in-place normalized string comparison

[stevengj]

See the discussion in #101 and the Julia implementation in https://github.com/JuliaLang/julia/blob/9d6344a6673387a4ac0ae04f197ab60055d7a05d/stdlib/Unicode/src/Unicode.jl#L268-L344 … one implementation was suggested in #298.

[LB--]

I'm still around to work on #298 but nobody replied to give feedback on my latest iteration of the API design and implementation, so I'm not sure whether to proceed with adding tests or if changes are needed.