core: optimise Debug impl for ascii::Char

Rather than writing character at a time, optimise Debug implementation
for core::ascii::Char such that it writes the entire representation as
with a single write_str call.

With that, add tests for Display and Debug implementations.

library/core/tests/lib.rs

@@ -122,6 +122,7 @@ mod alloc;
 mod any;
 mod array;
 mod ascii;
+mod ascii_char;
 mod asserting;
 mod async_iter;
 mod atomic;