std::ascii: Provide a copyless [Ascii] -> str method.

This renames to_str_ascii to as_str_ascii and makes it non-copying,
which is possible now that strings no longer have a hidden extra
byte/null terminator.

Fixes #6120.
auto merge of #10333 : huonw/rust/ascii, r=alexcrichton

This renames to_str_ascii to as_str_ascii and makes it non-copying,
which is possible now that strings no longer have a hidden extra
byte/null terminator.

Fixes #6120.