Block, Output and Kernel now have bitmasks to hold supported
features and eventually versioning. Will make adding features and
updates easier and open the possibility of soft forks.
First added feature for Output and Kernel is the marking of coinbase
related ones. Allows the validation of the coinbase part of a block.
Documentation aimed at a technical audience, just requiring some basic
knowledge of bitcoin, to explain how MimbleWimble works and how it's
applied in Grin.
