Explore support for a bit-level cursor

[jverkoey]

The encoding and decoding implementations presently use a byte-level cursor. If bit coding is desired, however, the byte granularity makes requires that bits are packed and unpacked into bytes before being coded. This creates a fair amount of overhead for tightly packed data formats.

A successful solution to this task will meet the following requirements:

• [ ] Both the encoder and decoder provide APIs for working with a bit-level cursor.
• [ ] There is a clear relationship between bit-level cursor and byte-level cursor.

One possible approach is to provide a bit container view.