This patch fixes some minor typos in comments and strings found by
codespell.
While at it, also expand some variable names that were not typos, but
caused false positives, and end up being more readable anyway.
This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.
In some cases, the identifiers were un-exported after noticing they had
no external users.
Besides improving documentation, it also reduces the linter noise
significantly.
We will need a few string sets in different places, create it as a separate
package for convenience.
For now, it only implements strings, but it may be extended to other data
types in the future.
