Rollup merge of #141445 - yotamofek:pr/library/from-iter-char-string, r=the8472,joshtriplett

Add `FromIterator` impls for `ascii::Char`s to `String`s

Wanted to `collect` ascii chars into a `String` while working on #141369 , and was surprised these impls don't exist. Seems to me to be simply oversight.

BTW, I only added `impl FromIterator<ascii::Char> for Cow<'_, str>`, without a corresponding `FromIterator<&Char>` impl, because there's no existing impl for `FromIterator<&char>`, but that might be oversight too.

cc #110998

Author: matthiaskrgr