Make function names terse. Make functions private.

Rename all the generator functions. Use the prefix "gen_" instead of
"generate_". For example, `generate_date` is now `gen_date`.

Make helper functions private. Rename `codify` to `_make_unicode` and
`is_positive_int` to `_is_positive_int`.

Silence some pylint and flake8 warnings, either by fixing the relevant error
(e.g. use better names for unused variables) or silencing the relevant error
(e.g. the use of the Python 2 keyword "unicode").