cram: Define bit stream value

[zaeleus]

This is in regard to CRAM format specification (version 3.1) (2024-09-04).

§ 2.2 "Writing bits to a bit stream" does not define what a value is. This is required to know how to interpret the bits and the maximum number of bits that can be written to the stream from said value.

A single value type can be generally defined for the bit stream (e.g., int32_t >= 0), or the encodings that use the bit stream (e.g., beta) can describe their own appropriate value types.