Loosen dependency on pretty (for recent Haskell platforms).
Support system F.
Support argument syntax for top-level bindings.
Merge pull request #5.

Conflicts:
	pts.cabal
Make tests compile again

To do so, add needed imports and update tests to use the new equality
definition. The test now fail when comparing terms with division by
zero - the division is not residualized.
Normalisation fails on division by zero: add testcase
Remove dead code to not maintain it

Otherwise, this code would stop compiling with next commit.
Fix normalization: residualize division by zero