Cache Native/Overide alt lookups.

Looking at a startup profile, the following checks stand out within VM.execute:

- Override.hasMethod (450ms)
- ACCESS_FLAGS.isNative (400ms)

Caching the method lookup avoids these checks.
> capital names for variables are weird, how about just "alt"?

Indeed. I originally called this *Alt* because it was set to either *Native* or *Override*, which are both capitalized. But now that it gets set to *methodInfo.alt* it makes more sense to make it all-lowercase.

(You could also make *methodInfo.alt* a memoizing unary getter that returns the native/override function itself, such that this code merely has to check `if (methodInfo.alt)` before calling it. Unsure about the performance characteristics of that approach.)