compiler: Replace FindMutableVars with FindVars

This will be used by the abstract interpreter to perform optimisations
and inlining based of on constant values
stdlib: Implement integer?, fix exact?/inexact?

exact? and inexact? are defined only to take numbers but our stdlib
defined them as taking any object. Fix them to only accept numbers.

Add integer? which works the way exact? used to - accepting any object
and testing if it's an integer.