Remove bone-keyed caches in bone extension

[Denneisk]

• Removes bone2index
• Removes bone2entity
• Reformatted tostring(bone) function. Now operates similarly to Gmod's physobject tostring.
• Revert bone:index() function
• Change signature of E2Lib.getBone to return only an entity now

entity2bone still exists because it might be more efficient. Worst case, it'll recache it and that's not that bad.

Fixes #3018

I miss being able to show the index in printing :(