A type alias.

An type alias can either be holey or filled.

I believe that holey type aliases are those with generic parameters.

An elm type annotation.

The set of strings are the free type variables that the annotation contains.

A field in an elm record with an index and a type.

A type definition or alias in elm.

An elm custom type.

This record stores the type variables in vars and the "options that you would write a case statement for" in alts.

Note: the binary data also includes the number of alternatives and some optimation information - this is all currently discarded.

A contructor for a custom type alternative.

TODO: document the meaning of each arg.

Note: some of these ints are redundant.