ZeroTrie types simplification

[sffc]

Currently we have 4 types:

• ZeroTrieSimpleAscii
  • Supports ASCII only (so that it can be const constructed)
  • Does not use the perfect hash function (better for small data)
  • Does not support span nodes (because ASCII does not need them)
  • Maximum size is 2^32
• ZeroTriePerfectHash
  • Supports arbitrary bytes, including UTF-8
  • Uses the perfect hash function for branch nodes with at least 16 children
  • Uses span nodes
  • Maximum size is 2^32 (slight performance win)
• ZeroTrieExtendedCapacity
  • Same as ZeroTriePerfectHash but there is no size limit
• ZeroTrie
  • An enum over the three above types

I think we should consolidate these down to two types:

• ZeroAsciiTrie == ZeroTrieSimpleAscii
• ZeroBytesTrie == ZeroTrieExtendedCapacity
• Remove ZeroTriePerfectHash because the ~5% perf benefit isn't worth keeping around a separate type
• Remove ZeroTrie because the type of trie should be determined by the programmer based on the nature of the data, not via a runtime switch

OK?

• [x] @Manishearth
• [x] @robertbastian

[sffc]

Worth noting that the byte pattern of a ZeroAsciiTrie can be consumed by a ZeroBytesTrie which means it is forwards-compatible to switch between them.