Consider exposing SignatureComparer

[kzu]

Consider the scenario where a source generator must check for the presence of a certain member by signature (i.e. to determine if the class implements a certain interface already, or has a given convention-based member) to determine whether certain code generation must happen or not.

Currently, the SignatureComparer is an internal shared utility class, which prevents this scenario.

Note: attempting to extract just this class from Roslyn' source is a non-trivial endeavor since it has dependencies on the internal SymbolEquivalenceComparer class and all its nested classes.