Collect types also for non-monadic functions

Primitive operations in C world, such as `==`, `&&`, etc. are naturally
represented as non-monadic functions in Gallina.

With this patch, given this code:

    do x <- returnM (eqb y z) ;
    ...

the type for x is properly generated as the return type of eqb, without
requiring an explicit definition of a monadic lifting of eqb.