case-conv

Fast and correct case conversion.

Provides a fast case for ascii, but falls back to a unicode correct version if it discovers non ascii chars.

Benchmark results

Tests performed on a Ryzen 5 3600X on Linux

* means it's from this crate. Lower number is better

``` lowercase/ascii time: [947.44 ns 956.89 ns 971.81 ns] * lowercase/unicode time: [1.5051 us 1.5081 us 1.5116 us] * lowercase/asciistd time: [2.1723 us 2.1797 us 2.1887 us] lowercase/unicodestd time: [2.3111 us 2.3155 us 2.3205 us]

uppercase/ascii time: [938.50 ns 944.61 ns 952.93 ns] * uppercase/unicode time: [1.3386 us 1.3436 us 1.3496 us] * uppercase/asciistd time: [1.8429 us 1.8555 us 1.8687 us] uppercase/unicodestd time: [1.9726 us 1.9761 us 1.9800 us] ```