instead of converting from ssz -> struct -> ssz, it may be better to
just stay as ssz, then use methods to read the data.
this pr explores this concept, while maintaining compatiblity with the
existing codebase.
this first major move separates the transient beacon state cache from
the underlying tree.
leaf updates are enforced in the setters, which should make programming
easier.
all exported methods of the raw.BeaconState should be safe to call
(without disrupting internal state)
changes many functions to consume *raw.BeaconState in perparation for
interface
beyond refactor it also:
adds a pool for the leaves of the validator ssz hash
adds a pool for the snappy writers
removed the parallel hash experiment (high memory use)
