Implement a simple heuristic for typing lists: we compute the sup of the types of its elements. This is not as general as it should be since evars might be substituted during the computation of the sup (it is however correct). However, usual examples hitting this limitation are now being typed.

--HG--
branch : LS-551
Revert David's quick hack patch since it's not needed anymore in this branch.

--HG--
branch : LS-551
Merge branch 'master' into LS-551

Conflicts:
	src/operators/noblank.ml
Mistakenly added back skip_blank while merging.
Add debug message
Correct subtyping order.
Merge pull request #126 from savonet/LS-551

Better list subtyping