Fix: reference to an overloaded function

Fixes bug #202 by raising a `AmbiguousFunctionReferenceException` when
trying to reference an overloaded function without providing arity,
using ether `fun(name, module)` or the `^` shorthand.
Code formating
Merge pull request #203 from yloiseau/fix/RefToPolymorph

Fix: ambiguous reference to an overloaded function