mp: Generalize function information retrieval

Generalizes MemFnInfo to be compatible with all function types.
Also adds type introspection for arguments, as well as helper templates for the common types supported by all partial specializations.

src/frontend/decoder/arm.h

@@ -22,7 +22,7 @@ namespace Arm {
 
 template <typename Visitor>
 struct ArmMatcher {
-    using CallRetT = typename mp::MemFnInfo<decltype(&Visitor::arm_UDF)>::return_type;
+    using CallRetT = mp::return_type_t<decltype(&Visitor::arm_UDF)>;
 
     ArmMatcher(const char* const name, u32 mask, u32 expect, std::function<CallRetT(Visitor&, u32)> fn)
             : name(name), mask(mask), expect(expect), fn(fn) {}