This eliminates the possibility of infinite loops caused by counters
overflowing and preventing the output buffer full or input exhausted
conditions from being reached.

Previously, suspending between the two bytes of a >8-bit index or count
could lead to accumulation of an incorrect value. This commit breaks
each of the index and count states into X_{msb,lsb}, where _msb reads
the >8th bits and stores them in the accumulator, and _lsb reads the
lower 8 bits (or all of them, if the total value is < 8). This adds
another point where the state machine can suspend and resume without any
loss of information, if necessary.

Since they're only stored as `uint16_t`s, disallow overly large sizes.