Take function pointer types from the AST instead of parsing type strings

[guitargeek]

The function pointer branches of CreateExecutor() and CreateConverter() sliced the printed type string (e.g. "int (*)(double, char)") to recover the return type and the argument list, as flagged by a "find better way of finding the type" TODO. Replace this with a new interop::GetFnTypeSigAsString(), built on GetFnTypeSig(), which asks clang for the pointee function prototype's return and parameter types. GetFnTypeSig() now guards against non-prototype types, as Cpp::GetFnTypeSignature() crashes on those.

This fixes two bugs of the string parsing:

• the type-based executor path removed all spaces before slicing, so any multi-word return type such as unsigned int (*)() broke with an attempt to compile std::function<unsignedint()>;
• member function pointers matched the "::*)" check and produced a bogus std::function. They are not function pointers and are now rejected (returning one yields None as before, a data member of that type is reported as unconvertible).

The string-based overloads keep the type from the lookup that ResolveName() performed anyway, and the regex used by the string-based CreateConverter() is dropped.