Optimize String#to_u methods for the case of a negative number

[wooster0]

This makes String's to_u64_info method return early when the number that should be converted to an unsigned integer is negative.

Benchmark.ips do |bm|
  bm.report("old") do
    "-112978314788778".to_u64 { false }
  end

  bm.report("new") do
    "-112978314788778".new_to_u64 { false }
  end
end

old  58.13M (  17.2ns) (± 5.40%)  0 B/op  10.21× slower
new  593.7M (  1.68ns) (± 4.43%)  0 B/op        fastest

---

Benchmark.ips do |bm|
  bm.report("old") do
    "-112978314788778913431478989314731437891343734789943413789474187178137914713173484137134780".to_u16 { false }
  end

  bm.report("new") do
    "-112978314788778913431478989314731437891343734789943413789474187178137914713173484137134780".new_to_u16 { false }
  end
end

old  43.73M ( 22.87ns) (± 8.58%)  0 B/op  13.42× slower
new 586.69M (   1.7ns) (± 3.40%)  0 B/op        fastest

[straight-shoota]

I don't think the early return is worth merging.

[RX14]

It improves performance in an edge case only, but that doesn't mean it's not worth merging, as it's only a small bit of code.