Allow under-constrained function applications

[Alex Knauth]

Under-constrained function applications are applications where the function can be partially instantiated, but there are extra type variables in the return type that are un-constrained, meaning it would be valid no matter what they were. Expressions like this would partially instantiate the forall type so that it could be fully instantiated (with any type(s)) later.

[Stephen Chang]

eg, partially applied curried functions

[Alex Knauth]

This keeps getting re-closed because of a commit on the partial-inst branch, but this should be kept open until it's resolved on the master branch.

[Alex Knauth]

modify #%app to deal with under-constrained function applications

fixes #16

→ <<cset 9920c5791d45>>

[Alex Knauth]

It turns out that this can lead to unsoundness in the cases where both mutation is involved and the introduced foralls are instantiated more than once with different types.

[Alex Knauth]

This must be solved in a way that does not allow the unsoundness mentioned in https://bitbucket.org/stchang/macrotypes/issues/16/allow-under-constrained-function#comment-27692493.

[Alex Knauth]

This could still be improved by allowing type variables that can only be instantiated once.

[Stephen Chang]

fix #16

→ <<cset 39be2ef9044c>>