Add 8-bit states to ac-tile

When running with sgh-mpm-context: full, many more MPMs are created
(16K) and many are small. If they have less than 128 states, they only
need 1 byte for the next state instead of 2 bytes, cutting the size of
the next-state table in half. This reduces total memory usage.

Since that makes 3 different state sizes (1, 2 and 4 bytes), rather
than going from 2 copies of the code to create the MPM to 3, I
factored out the code that fills the next-state table into three
functions so that all the other code could be the same.

The search function is now parameterize for 8-bit and 16-bit state
sizes and alphabet sizes 8, 16, 32, 64, 128 and 256.