Get rid of the CFUNC overhead in String#ascii_only? and #valid_encoding?. Define them as leaf Primitive.cexpr! builtins in a new string.rb, following the pattern of numeric.rb.
This makes them 1.2x-1.4x faster across the interpreter, YJIT, and
ZJIT.
Move ZJIT's cfunc annotation for String#ascii_only? to annotate_builtin!, and add one for String#valid_encoding?.
Fix CI for builtin String#ascii_only?
