Various optimizations in RLE routines
Remove special case for lone 0x9B from RLE

The special case was too slow to justify the small increase in
compression it gave.

This commit addresses #16.
Implement rlePredictDecompress

Again, there are no possible errors at the moment.

This commit addresses #16.
Implement rlePredictCompress

I found it easier to just reuse a lot of the code from rleCompress.

This commit addresses #16.
Clarify RLE error codes

That is, there are none.  Yet.  Once stuff like overlapping
compression and reading/writing streams is enabled, there will be
suitable errors in place.

This commit addresses #16.
Implement RLE unit tests

This commit addresses #16.