Remove code duplication

[sol]

Use unsafeDrop and friends instead of explicitly constructing values with Data.ByteString.Internal.BS.

• All those primitives are marked with INLINE.
• This does not change the generated core.

Underlying motivation:

Zero-copy conversion from ByteString to Text and O(1) conversion from Text to ByteString offer substantial benefits. Despite #193 being closed, I still think it's desirable that the code makes it easier to experiment with different underlying representations. If at the same time, this reduces code duplication and makes the code easier to read, then the better.

I only looked at construction sites for now. There are more low hanging fruit, but before tackling those I want to make sure that we are on the same page.

[sol]

could you please run benchmarks before and after? Ideally using https://github.com/Bodigrim/tasty-bench/blob/master/compare_benches.sh.

All
  Data.ByteString.Builder.Prim
    int64Host (10000):                               OK
      4.10 μs ± 364 ns,  8% less than baseline
  Data.ByteString.Builder.Prim.ASCII
    intersperse (unaligned):                         OK
      983  ns ±  49 ns,  5% less than baseline
  folds
    strict
      foldr'
        65536:                                       OK
          37.6 μs ± 1.2 μs,  3% less than baseline
      unfoldrN
        65536:                                       OK
          609  μs ±  52 μs, 11% more than baseline
      mapAccumR
        1:                                           OK
          15.6 ns ± 678 ps, 10% less than baseline
        8:                                           OK
          17.6 ns ± 658 ps,  9% less than baseline
        64:                                          OK
          49.8 ns ± 2.3 ns, 12% less than baseline
      scanr
        8:                                           OK
          14.3 ns ± 394 ps, 11% less than baseline
    lazy
      foldl'
        128:                                         OK
          65.9 ns ± 6.5 ns, 19% more than baseline
        65536:                                       OK
          223  μs ± 4.7 μs, 10% less than baseline
      foldr1'
        4:                                           OK
          20.1 ns ± 1.1 ns,  9% less than baseline
        8:                                           OK
          39.3 ns ± 2.8 ns,  9% more than baseline
        64:                                          OK
          251  ns ± 7.1 ns,  9% less than baseline
        1024:                                        OK
          3.59 μs ± 285 ns,  9% less than baseline
        65536:                                       OK
          224  μs ± 8.4 μs,  7% less than baseline
      mapAccumR
        4096:                                        OK
          53.1 μs ± 3.6 μs,  9% less than baseline
  findIndexOrLength
    break:                                           OK
      30.1 μs ± 2.7 μs,  9% more than baseline
  unlines
    lazy:                                            OK
      128  μs ± 3.8 μs,  6% less than baseline
  BoundsCheckFusion
    Data.ByteString.Builder
      foldMap (right-assoc) (10000):                 OK
        105  μs ± 6.3 μs, 13% more than baseline
      foldMap [manually fused, right-assoc] (10000): OK
        108  μs ± 2.8 μs, 13% more than baseline
  Count
    no matches, same char
      1030 chars long:                               OK
        87.0 ns ± 4.8 ns,  6% less than baseline
    some matches, short cycle
      1030 chars long:                               OK
        86.8 ns ± 4.2 ns,  8% less than baseline
    all matches
      1030 chars long:                               OK
        86.2 ns ± 6.4 ns,  9% less than baseline
  Indices
    ByteString index equality inlining
      FindIndices/inlined:                           OK
        14.6 μs ± 775 ns, 12% less than baseline
  Read Integral
    Strict
      ReadInt:                                       OK
        4.50 μs ±  83 ns, 18% less than baseline
    Lazy
      ReadInt32:                                     OK
        4.00 μs ± 175 ns, 11% less than baseline
      ReadWord64:                                    OK
        5.35 μs ± 301 ns,  9% less than baseline
  ShortByteString
    intercalate
      intercalate (large):                           OK
        2.71 μs ±  71 ns,  5% more than baseline
    folds
      strict
        foldl'
          32:                                        OK
            22.0 ns ± 2.1 ns, 30% more than baseline
          64:                                        OK
            34.9 ns ± 2.8 ns, 18% more than baseline
        foldr1'
          64:                                        OK
            35.8 ns ± 3.5 ns, 15% more than baseline
    findIndex_
      findIndices:                                   OK
        4.62 μs ± 141 ns,  3% less than baseline
    ShortByteString strict second index
      FindIndex:                                     OK
        29.8 μs ± 1.1 μs,  9% less than baseline
    ShortByteString conversions
      unpack
        1:                                           OK
          10.0 ns ± 766 ps,  9% more than baseline
        64:                                          OK
          259  ns ±  22 ns, 11% more than baseline
        128:                                         OK
          518  ns ±  43 ns, 13% more than baseline
        2048:                                        OK
          7.42 μs ± 619 ns,  8% less than baseline
      pack
        2048:                                        OK
          7.13 μs ± 689 ns, 75% less than baseline
        4096:                                        OK
          17.1 μs ± 958 ns, 70% less than baseline
        16384:                                       OK
          63.5 μs ± 4.8 μs, 61% less than baseline
        32768:                                       OK
          252  μs ±  10 μs, 98% more than baseline