Handle `define-type`

[bennn]

require/typed/scv needs to generate contracts from ALL types, including user-defined types.

Not sure what to do about this

• Run the Typed Racket typechecker, get its type environment?
• Parse the program for define-type, get the surface syntax?

[bennn]

Running the typechecker seems hard (not possible until after expansion)

Let's try surface syntax, and require that any define-type come before the relevant require/typed/scv.

• define a macro define-type
• store aliases in a free-id-table
• use the table in the (still simple) type-syntax->contract function