Support TypeApplications

It would be nice to support the TypeApplications language extension. But I don't know if backporting as for ApplicativeDo (#120) is feasible.

So far it already causes non-trivial patching of Vinyl (https://github.com/typelead/eta-hackage/blob/master/patches/vinyl-0.8.1.1.patch) and I'd expect more libraries to use TypeApplications in the future.

@dkellner TypeApplications is very much a priority for us, just need to get around to port it. Before taking a stab at it, we need to:

1. Deprecate the @-wrapper syntax because it clashes with TypeApplications and it's a weird syntax hack anyways. This will be done as soon as #647 is implemented which is currently being worked on.
2. Make sure all the typechecker tests in the GHC testsuite are included so that if we encounter regressions after doing the port, we can catch them quickly. TypeApplications does some non-trivial changes to the typechecker algorithm and a simple mistake in the port can cause a cascade of other errors.