Implement type specialized instance variables

[alex]

What does this mean @a = <some int or float> doesn't allocate anymore. Todo list:

• [x] bring back pre-sizing of the data array (and tests for it)
• [x] make pos() not be O(n) in the number of instance vars.
• [x] merge precise-instantiate branch of PyPy.
• [x] investigate how much of a pain merging the coerce_bytes resops into the getarrayitem_gc would be (https://bitbucket.org/pypy/extradoc/commits/6afbc6bbdc37a38f90b9ddea1ee8681483b36f19).