`lower`, `upper` could be further optimized for ASCII-only inputs

[neilconway]

Is your feature request related to a problem or challenge?

We do the case conversion via str::to_lowercase() / str::to_uppercase(); if we've already checked that the input is ASCII-only, we can use str.to_vec().make_ascii_lowercase() instead.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[neilconway]

take