various improvements to parser (#22161)

- move call to `short-form-function-loc` inside `parse-assignment`
  this is faster and fixes nested cases like `f(x)=g(x)=1`
- fix a case where an `unexpected "foo"` message was missing quotes

- optimize common case of arglists of comma-separated simple tokens
- some other small optimizations

- put code back in precedence order
- separate parse-Nary and parse-comma so they're easier to read
- cleaner code for parse-unary and parse-factor