Add a notion of type equality in the client

See merge request !44

These comments should appear in F.mli too, but I'm waiting for feedbacks before I copy-paste them.

The term Refl is a proof of a type equality TyEq(ty1,ty2) and Use is a construct that introduce a type equality in the typing context.

This will be useful when we translate System F terms to graphs (a "structure" node of the graph will be represented by a Structure.structure).

Compact syntax for repeated abstractions and type abstractions.

See merge request !45

Fix a bug (incorrect assumption when using [letr1] in the client).

See merge request !43