Now that we generate methods in instantiated generic types, we were
getting test failures from methods that were being detected as new
methods. In actuality, they weren't new, but they differed only in
generic type parameters from some base type method, and
GetSignatureString ignores generic parameters completely.
This fix eliminates the hacky GetSignatureString and replaces it with
more-or-less proper signature comparison. This even manages to fix an
incorrect test case from Methods.cs (because GetSignatureString was
incorrectly incorporating the return type - when the return type should
not be examined for signature checking).
We adopt roughly the same approach as the C# Reflection API: a
GenericMethodDefinition is a method which has no method parameters
substituted, but lives in a open or closed type with some or all
type parameters substituted. To ensure the uniqueness of the MethodInfo,
we cache by the method type arguments, and also cache generated
DeclaredConstructors/DeclaredMethods in the TypeInfo.
This also enables MakeGenericMethod, albeit in a slightly different form
than the Reflection API: MakeGenericMethod lives in MethodBase, so it's
callable from a constructor (even though in C# constructors cannot be
generic). This slight violation of the spec reduces code duplication, so
it's probably worth it.
Finally, VirtualAddress gets set when populating GenericMethods, and so
it'll work whether or not the methods get cached/generated ahead of
time.
