Write macro to help the parse-type function

[bennn]

The syntax->type-rep function converts a Typed Racket syntax object to syntax for racket/contract. It has a few maintenance problems:

• uses #:literals to match Typed Racket identifiers, it's easy for #:literals to diverge from the syntax-parse patterns
• there's lots of repetitive patterns, like Listof and Vectorof are very similar cases in the "fold" over TR syntax
• the output might not be valid racket/contract syntax