This follows in the footsteps of #573, where we introduced a standard IInvocation implementation type and re-used it for all methods of interface proxies without target. This present PR does the same for abstract methods of class proxies without target: those would always get a custom-built invocation type with identical InvokeMethodOnTarget implementations; so they, too, are eligible for invocation type reuse.

The tests cover all combinations of (a) abstract vs. virtual, (b) public vs. protected, and (c) generic vs. non-generic method:

• (a) is in fact the only distinction that really matters.
• I've included (b) because of EmitCallThrowOnNoTarget apparently depends on the presence of a callback method, and protected would regularly lead to callback methods being emitted.
• I've included (c) i.e. generics mainly because I did so in Speed up interface proxy w/o target type generation significantly #573 (even though TBH I cannot quite remember why I thought it necessary back then).