Using `c.is_uppercase()` instead of getting the iterator and checking if the
first (and only) lowercase letter of the sequence is the same as the original
input is 5-8x faster (measured via criterion against `/usr/share/dict/words`).
(Additional benefit of forcibly inlining the now iterator-based comparison not
taken into account; this necessitated changing from a closure to a local
function as the inline attribute on closures is not yet supported with the
stable compiler toolchain.)
It is short and simple enough to write yourself if you need it and it encourages
bad behavior by a) always returning owned strings, b) always allocating them in
a vector. If/where possible, it is better to a) use &wstr, b) use an iterator.
In rust, it's an anti-pattern to unnecessarily abstract over allocating
operations. Some of the call sites even called split_string(..).into_iter().
This function is a hotspot, but it has inefficient codegen:
1. For whatever reason, the chars() iterator of wstr is slower
than that of a slice. Use the slice.
2. Unnecessary overflow checks were preventing vectorization.
Switch to a more optimized implementation.
This improves aliases benchmark time by about 9%.
