Rollup merge of #120314 - mina86:i, r=Mark-Simulacrum

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
with a single write_str call.

With that, add tests for Display and Debug.

Issue: rust-lang/rust#110998

Author: matthiaskrgr