Relax isExpressionFresh and improve aliasing error messages

[mario-bucev]

Relax the condition over which an expression is considered to be "fresh" to include bindings of match expression and seeing through non-recursive function.
It also changes the rule for Let for which I'm not so sure about:
Before: Let(vd, e, b) expression fresh iff e is fresh and b as well, where we consider vd as fresh when recurring on b
Now: Let(vd, e, b) fresh iff b is fresh, where we add vd as a fresh binding when recurring on b provided e is fresh.
These changes allows to accept FreshExpr.scala (which was rejected before).

It also improves a bit the "illegal aliasing" error messages.