Improve inlining of Klass accessors

When linking classes, significant time is spent in klassVtable::initialize_vtable, which calls into Klass::vtable, vtable_start_offset etc. These accessors compile down to what is essentially a few instructions, so moving them to klass.inline.hpp an effective optimization: more or less footprint neutral, and removing calls in numerous places. Other oft-used accessors like Klass::class_loader are also beneficial to inline.

In one startup test, this reduces instructions retired in initialize_vtable from 88M to 66M alone, while also improving many other call sites to the tune of about a 1% net reduction in instructions retired overall.