Add a new DHGRBitmap class that efficiently represents the
DHGR interleaving of the (aux, main) MemoryMap as a sequence of
28-bit integers.
This allows for easily extracting the 8-bit and 12-bit subsequences
representing the DHGR pixels that are influenced when storing a byte
at offsets 0..3 within the interleaved (aux, main, aux, main)
sequence.
Since we have precomputed all of the pairwise differences between
these 8- and 12-bit values, this allows us to efficiently compute the
edit distances between pairs of screen bytes (and/or arrays)
