Avoid defining singleton methods

[nevans]

This PR only creates new singleton methods when they are needed for overwriting an existing instance method. It should be mostly backward-compatible with the current OpenStruct implementation, but I suspect there are some minor incompatibilities (which may be significant).

I believe this should generally be a significant performance improvement. I'm guessing that OpenStruct objects are generally ephemeral and their attributes are generally read fewer than a dozen times on average. In my benchmarks, that wasn't enough for the benefits of defining singleton methods to outweigh the costs. All of the benchmarks I created, except for 100x reads, showed the same or better performance.

And that's mostly ignoring any non-local costs to constantly busting method caches. This PR should almost completely remove that cost.

$ benchmark-driver benchmarks/ostruct.yml
Warming up --------------------------------------
                  new     7.047k i/s -      7.711k times in 1.094212s (141.90μs/i)
      attr_write_read     4.904k i/s -      4.910k times in 1.001289s (203.93μs/i)
index_write_attr_read     6.346k i/s -      6.963k times in 1.097167s (157.57μs/i)
           null_reads    79.488k i/s -     85.250k times in 1.072495s (12.58μs/i)
            10x_reads     6.651k i/s -      7.282k times in 1.094836s (150.35μs/i)
           100x_reads     5.136k i/s -      5.150k times in 1.002780s (194.71μs/i)
Calculating -------------------------------------
                          v0.6.0       local
                  new     6.938k     33.067k i/s -     21.141k times in 3.047342s 0.639334s
      attr_write_read     4.886k     14.021k i/s -     14.711k times in 3.010686s 1.049232s
index_write_attr_read     6.270k     20.225k i/s -     19.039k times in 3.036489s 0.941373s
           null_reads    78.351k     77.187k i/s -    238.462k times in 3.043491s 3.089414s
            10x_reads     6.518k     13.306k i/s -     19.953k times in 3.061390s 1.499576s
           100x_reads     4.454k      2.078k i/s -     15.407k times in 3.459125s 7.414447s

Comparison:
                               new
                local:     33067.2 i/s
               v0.6.0:      6937.5 i/s - 4.77x  slower

                   attr_write_read
                local:     14020.7 i/s
               v0.6.0:      4886.3 i/s - 2.87x  slower

             index_write_attr_read
                local:     20224.7 i/s
               v0.6.0:      6270.1 i/s - 3.23x  slower

                        null_reads
               v0.6.0:     78351.5 i/s
                local:     77186.8 i/s - 1.02x  slower

                         10x_reads
                local:     13305.8 i/s
               v0.6.0:      6517.6 i/s - 2.04x  slower

                        100x_reads
               v0.6.0:      4454.0 i/s
                local:      2078.0 i/s - 2.14x  slower