Fix the Type::Function variant

[Marwes]

Currently the Function variant has holds a Vec<Type> for function arguments but it is always assumed that the vector only has one element as functions are curried and are always treated as 1-argument functions.

The variant should either just hold one argument type, mirroring how the type checker sees it, or the code should take advantage of the Vec to store multiple arguments (possibly for efficency's sake).

[Marwes]

Type::Function was removed in #55