Some names in the internal implementation of the linker could be improved.

I suggest the following renaming changes.

  - ProgrammableInvoker -> DowncallLinker
  - ProgrammableUpcallHandler -> UpcallLinker
  - 'native invoker' -> 'downcall stub'
  - 'optimzed upcall stub' -> 'upcall stub'
  - OptimizedEntryBlob -> UpcallStub
  - optimized_entry_frame -> upcall_stub_frame

Then renaming of some hotspot files:

  - universalNativeInvoker* -> downcallLinker*
  - universalUpcallHandler* -> upcallLinker*
  - foreign_globals* -> foreignGlobals* (to match existing convention)