sumtype: reduce compile-time overhead of has & get

[pbackus]

To reduce the number of template instantiations per call, this change replaces the original match-based implementations of has, get, and tryGet with specialized implementations that inline both the matching and the handlers into a single function body.

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[thewilsonator]

I trust you have run the numbers for this?

[pbackus]

I got the following results, using a SumType with 5 members, counting instantiations with -vtemplates, and looking only at templates from std/sumtype.d:

• has:
  • Before: 49 instantiations per call (27 distinct)
  • After: 4 instantiations per call (2 distinct)
• tryGet
  • Before: 54 instantiations per call (32 distinct)
  • After: 12 instantiations per call (8 distinct)

I assume get is the same as tryGet because they share their implementation.

I haven't attempted to measure time or memory consumption yet. If we want those numbers, let me know, and I'll do so.