Go: Finish implementing the encoding path through cmd.entropylex.EntropyLex8

[icahoon]

• Write the encode path through the cmd.entropylex.EntropyLex8.
• Write thorough unit tests.
• The entropylex8 encoding/encode has yet to be written, so the implementation should abstract the behavior, and thus mock that behavior in unit tests.
• For now use 1k buffers to read from the input io.