Commits for details.
This shouldn't change the generated code at all (except for switching to `LitBinary` from an explicit ExprVec of individual ExprLit bytes for `prefix_bytes`).
char literals now work in a quotation.
There were several instances of duplicated functionality in regex_macros
compared to AstBuilder so refactor those out.
The AST doesn't need ~s everywhere, so we can save allocations this way
& the enum isn't particularly large (~4 words) nor are regexes
long (normally), so the space saved in the `Cat` vector is unlikely to
be very much.
