Add a Bitstream Writer Utility

This is needed to emit priors for cross-module dependencies in the new Swift driver.

Harlan Haskins has graciously donated the baseline implementation of the Bitstream writer from harlanhaskins/swift@bf9f98d#diff-1c6090ba4f05ee6c5f695c55a71f287c6234aa84997d63472d1320c9892730e7R39

I have modified its API surface to include a literate API for writing records, adapted some of its internal structure to make it more compatible with the existing abstractions, and added lots of unit tests.

Author: CodaFi