Improve perf of String::char_len and String::is_valid_encoding

[lopopolo]

Introduce bytecount and simdutf8 for SIMD-accelerated UTF-8 validation and character counting.

Performance

I've tested this PR out on the tip of the WIP PR to add String to the VM #1222. These numbers compare with and without this PR as well as against MRI 3.0.2.

This PR speeds up the existing implementation of String::length by a factor of 10 and brings String#length performance to the same order of magnitude as MRI in all cases except pure ASCII strings. MRI caches which valid encodings a String has. For ASCII Strings this fast path is very fast: it returns the length embedded in the RString.

Benchmark script string_length.rb

# frozen_string_literal: true

def bench(s, name:)
  s = s * 100_000
  before = Time.now
  result = s.length
  after = Time.now

  puts "#{name}; length: #{result}; time: #{after - before}"
end

s = "a" * 10_000
bench(s, name: 'UTF-8 only ascii')

t = s + '💎' * 10_000 + s
bench(t, name: 'UTF-8 with emojis')

u = 'a💎' * 10_000
bench(u, name: 'ascii + emoji interspersed')

v = 'a' * 10_000
v << '\xFF'
bench(v, name: 'invalid UTF-8 at tail')

w = s + '💎' * 10_000 + s
w << '\xFF'
bench(w, name: 'invalid UTF-8 at tail with emojis')

MRI 3.0.2

$ /usr/local/var/rbenv/versions/3.0.2/bin/ruby ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 2.0e-06
UTF-8 with emojis; length: 3000000000; time: 0.768025
ascii + emoji interspersed; length: 2000000000; time: 0.614303
invalid UTF-8 at tail; length: 1000400000; time: 1.0e-06
invalid UTF-8 at tail with emojis; length: 3000400000; time: 0.746079

$ /usr/local/var/rbenv/versions/3.0.2/bin/ruby ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 1.0e-06
UTF-8 with emojis; length: 3000000000; time: 0.739436
ascii + emoji interspersed; length: 2000000000; time: 0.624905
invalid UTF-8 at tail; length: 1000400000; time: 2.0e-06
invalid UTF-8 at tail with emojis; length: 3000400000; time: 0.75076

Without this PR

$ cargo run --bin artichoke --release -q -- ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 0.729179
UTF-8 with emojis; length: 3000000000; time: 7.913318
ascii + emoji interspersed; length: 2000000000; time: 7.228271
invalid UTF-8 at tail; length: 1000400000; time: 0.719593
invalid UTF-8 at tail with emojis; length: 3000400000; time: 7.944082

$ cargo run --bin artichoke --release -q -- ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 0.731228
UTF-8 with emojis; length: 3000000000; time: 7.867516
ascii + emoji interspersed; length: 2000000000; time: 7.173596
invalid UTF-8 at tail; length: 1000400000; time: 0.712632
invalid UTF-8 at tail with emojis; length: 3000400000; time: 7.86214

With this PR

$ cargo run --bin artichoke --release -q -- ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 0.06711499999999999
UTF-8 with emojis; length: 3000000000; time: 0.899454
ascii + emoji interspersed; length: 2000000000; time: 0.763296
invalid UTF-8 at tail; length: 1000400000; time: 0.064452
invalid UTF-8 at tail with emojis; length: 3000400000; time: 0.929182

[lopopolo]

with native target CPU:

$ RUSTFLAGS='-C target-cpu=native' cargo run --bin artichoke --release -q -- ../string_length.rb
UTF-8 only ascii; length: 1000000000; time: 0.064621
UTF-8 with emojis; length: 3000000000; time: 0.832497
ascii + emoji interspersed; length: 2000000000; time: 0.758753
invalid UTF-8 at tail; length: 1000400000; time: 0.063723
invalid UTF-8 at tail with emojis; length: 3000400000; time: 0.901933