ascii: Follow convention and return Option instead of failing

Use explicit unwrap() where needed. Basically rename:

* to_ascii_opt -> to_ascii
* into_ascii_opt -> into_ascii

and get rid of the version which uses fail!
Simplify is_ascii condition in the same way as is done for &str
Do not loose self in into_ascii() in case of error.

Return Err(self) from into_ascii() in case of error, so the original value is
not lost.
auto merge of #12320 : mneumann/rust/std_ascii, r=alexcrichton

Return an Option for to_ascii() and into_ascii() functions. Remove to_ascii_opt() and into_ascii_opt().