Add an "all" granularity to humanize

## Feature Request

Once #997 is implemented, using full precision would become more viable. However, it's currently cumbersome to specify that; it requires a list of all units. Sure, a wrapper can be made to abstract that away, but it'd be convenient to just pass the string `"all"`.

Review issue #997 first to understand the planned full-precision behavior, then trace the humanize granularity handling. Add support for the string "all" as the equivalent of specifying every unit, and verify the existing humanize behavior remains unchanged.

Written by the indexing model from the issue text.