Remove special case for id, type, and object_id

Mash overrides the definitions of these core object methods to allow users to
have keys with these names. While it works at first glance, this encourages
bad code that will eventually come back and bite the user. Possible weird
behaviors include:

* garbage collection `object_id`
* object identity comparison
* object type comparison

I won't merge this into the 1.x series since it's a breaking change, but I
plan to merge this after releasing 1.3

cc @mbleigh