Implement fast_float for String#to_f

[HertzDevil]

Resolves #11952.

This is a source port of https://github.com/fastfloat/fast_float, which is both locale-independent and platform-independent, meaning the special float values will work on MSYS2's MINGW64 environment too, as we are not calling LibC.strtod anymore. Additionally, non-ASCII whitespace characters are now stripped, just like #to_i.

The current implementation doesn't accept hexfloats.

Below is a basic benchmark that converts random Float32 strings:

require "benchmark"

f1 = 0.0
s = ""
f2 = 0.0

N = ENV["N"]?.try(&.to_i?) || 100000

Benchmark.ips do |b|
  b.report("ctrl") do
    N.times do
      f1 = rand(UInt64).unsafe_as(Float64)
      s = f1.to_s
    end
  end

  b.report("to_f") do
    N.times do
      f1 = rand(UInt64).unsafe_as(Float64)
      s = f1.to_s
      f2 = s.to_f64
    end
  end
end

Before:

ctrl  72.00  ( 13.89ms) (± 2.23%)  13.7MB/op        fastest
to_f  49.70  ( 20.12ms) (± 2.96%)  13.7MB/op   1.45× slower

After:

ctrl  72.81  ( 13.73ms) (± 2.37%)  13.7MB/op        fastest
to_f  63.39  ( 15.78ms) (± 2.56%)  13.7MB/op   1.15× slower

Thus fast_float brings the time down from 6.23ms to 2.05ms, a roughly 3x speedup, without any additional allocations.

[kostya]

I also added shard for this: https://github.com/kostya/fast_to_f