This re-working of the typechecker algorithm is based on
the paper "Visible type application", by Richard Eisenberg,
Stephanie Weirich, and Hamidhasan Ahmed, to be published at
ESOP'16.

This patch introduces -XTypeApplications, which allows users
to say, for example `id @Int`, which has type `Int -> Int`. See
the changes to the user manual for details.

This patch addresses tickets #10619, #5296, #10589.