Teach StringReader how to emit errors for arbitrary spans, so we don't
need to modify peek_span. This allows for emitting errors without having
a &mut borrow of the StringReader.
Move into a new syntax::parse::lexer::StringReader method the code
that was almost duplicated for parsing backslash-escapes in
byte, byte string, char, and string literals.
the leading quote part of the identifier for the purposes of hygiene.
This adopts @jbclements' solution to #14539.
I'm not sure if this is a breaking change or not.
Closes#12512.
[breaking-change]
