Perf: prefer non-allocating, case-insensitive string comparisons

[BastiDood]

Overview

Hello there! This PR removes the usage of the to_lowercase method in the sort module. I noticed that its goal was to perform case-insensitive pattern-matching, but it unfortunately requires an allocation (due to the String return type). The same goal may be achieved through the eq_ignore_ascii_case method, albeit with chained if conditions rather than a single match expression. The upside is that we no longer require the short-lived allocation.

Some Minor Changes in Behavior

I must note that --sort code and --sort CoDe (for example) will both still work. However, behavior will change if we give an invalid item such as --sort hello. Observe that the else block will consume all other cases. Hence, the printed error will exactly copy the incorrect input.

$ tokei ... --sort hello
Unsupported sorting option: hello

$ tokei ... --sort HeLlO
Unsupported sorting option: HeLlO

Previously, the to_lowercase method would have printed hello in all cases. I thought that this is just something worth noting during the review.

Anyway, that is all. Thanks! 🎉