remove unnecessary allocations in switch_case

[m4rch3n1ng]

previously it would allocate a Vec for every single char it would try to switch_case for, even when the case did not actually change, which is not necessary.

[nik-rev]

why even use flat_map? why not to_ascii_lowercase which returns char so we can just do a map

edit: oh, i understand that it's not guaranteed that we always have ASCII chars