fix: Allow unicode aware case insensitivity
fix: Allow unicode aware case insensitivity

In #2985, I noticed #2834 was incomplete, there were case-insensitive
comparisons we were doing without being unicode aware (when compile
options are set).

The downside is that each comparison will require a UTF-8 validation.
These seem to be in more of corners of the API, rather than in common
calls in common usages, so hopefully that isn't too much of a problem.

clap-rs/clap,clap-rs/clap,clap-rs/clap