Support sub-byte-resolution indices into dictionaries

[eyalroz]

The current implementation of dictionary compression assumes a dictionary entry index is some basic unsigned integral type; we can't have a dictionary of 128 entries, or 16, or 4.

We need to specialize for the case of sub-byte dictionary index - at least for units of 1, 2 and 4 bits if not for arbirtrary bit length (perhaps even over-1-byte).