Descriptive frame::metadata_words calculations

There are 37 places in the code that add, subtract or otherwise do something with frame::metadata_words in shared code. Having a couple of functions to abstract and describe why these calculations are needed would help a lot with understanding this code, and even potentially finding places where they should be added or subtracted that might be missing.

Plus, the name metadata_words in Hotspot suggest something that subclasses type Metadata (ie Klass, Method, etc), which these words are not. So we should also find a better name.