Refactor char/string and byte search

[jakobnissen]

This is a refactoring of base/string/search.jl. It is purely internal, and comes with no changes in behaviour. It's based on #54593 and #54579, so those needs to get merged first, then this PR will be rebased onto master.

Included changes are:

• The char/string search functions now use the last byte to memchr, not the first byte. Because the last bytes are more varied, this is much faster on small non-ASCII alphabets (like searching Greek or Cyrillic text) and somewhat faster on large non-ASCII ones (like Japanese). Speed on ASCII alphabets (like English) in unchanged.
• Several unused or redundant methods have been removed
• Moved boundschecks from the inner _search and _rsearch functions to the outer top-level functions that call them. This is because the former may be called in a loop where repeated boundschecking is needless. This should speed up search a bit.
• Much of this code used 0 as a sentinel value, possibly a leftover from before union-splitting was a thing. Replace this with nothing.
• Char/string search functions are now implemented in terms of an internal lazy iterator. This allows findall and findnext to share implementation, and will also make it trivially easy to implement a lazy findall in the future (see Implement lazy findall (Iterators.findall, perhaps?) #43737)

IMO there is still more work to be done on this file, but this requires a decision to be made on #43737, #54581 or #54584

Benchmarks

using BenchmarkTools
using Random

rng = Xoshiro(55)

greek = join(rand(rng, 'Α':'ψ', 100000)) * 'ω'
@btime findfirst('ω', greek)

@btime findfirst(==('\xce'), greek)

english = join(rand(rng, 'A':'y', 100000)) * 'z'
@btime findfirst('z', english)

@btime findall('A', english)
@btime findall('\xff', english)
nothing

1.11.0-beta2:

  100.049 μs (1 allocation: 16 bytes)
  474.084 μs (0 allocations: 0 bytes)
  689.110 ns (1 allocation: 16 bytes)
  93.536 μs (9 allocations: 21.84 KiB)
  72.316 μs (1 allocation: 32 bytes)

This PR:

  1.319 μs (1 allocation: 16 bytes)
  398.011 μs (0 allocations: 0 bytes)
  681.550 ns (1 allocation: 16 bytes)
  8.867 μs (8 allocations: 21.81 KiB)
  683.962 ns (1 allocation: 32 bytes)

[jakobnissen]

This is good to go now. Test failures are unrelated.