This is a major change to the way block metadata is handled within the PM core. This separates variant metadata (which really ought to be part of the ID) from state metadata, and in a couple of cases flattens separate states of blocks together.
The result of this is that invalid variants can be much more easily detected, and additionally state handling is much cleaner since meta is only needed at the serialize layer instead of throughout the code.
This now computes BBs relative to 0,0,0 and then offsets them as appropriate. This requires less boilerplate code and also furthers the goal of separating block types from instances.
