De-duplicate `tolower()`

Move the duplicated maptolower ASCII case conversion tables to isc_ascii where they can be shared, and replace the various hot-path tolower loops with calls to shared and newly-optimized isc_ascii implementations. My measurements suggest it is worth keeping BIND's own tolower table, because it is faster than <ctype.h> (presumably because it avoids the extra indirection required for locale support).

I have done a clean-up pass over the code to use <ctype.h> or isc_ascii rather than ad-hoc conversions.

I have found that LLVM will autovectorize a simple tolower() copying loop, but not a comparison loop, so the portable hand-vectorized tolower8() seems to be a win for longer comparisons. For (parts of) strings less than 8 bytes long I have kept the loops simple, to make it easier for the compiler and/or CPU to make them go fast.

(The main thing that seems to be missing from vector instruction sets and compiler intrinsics for this application is a way to load / store up to len bytes of a string into / from a register. It would make the tail end of strings much easier to deal with.)