Fix a performance bug that was occuring with programs having a lot of variables and that were using constant.

This is done via the new module EvalEq.

A lot of new errors appear in the on-reg tests, which is normal as
quite a lot of the code is now used...

Better error msgs.

Renaming idents.

Ident.t is now an abstract data type (instead of a string).
fullid is turned into an Ident.long, and is also made abstract.

Add basic regression tests.

Add "assert false" statements everywhere it was necessary.

Add the fby operator in the syntax.