Create sub modules `Time.[Protocol|System].Rfc3999` and move (+rename) RFC3999-related functions there

As discussed with @richard.bonichon and @raphael-proust over Slack + Mumble.

Current Time.[Protocol|System] modules have many RFC3999-related functions, some of which are ill-named (e.g. to_notation could really be named to_rfc3999_string), many of which are partial (more specifically for Time.Protocol as its domain is wider than the one of RFC3999).

This issue is about:

• Creating sub-modules Rfc3999 (or RFC3999?) in each of Time.Protocol and Time.System
• Move all related functions there
• Take the opportunity to improve the naming where applicable (e.g. s/notation/rfc3999)

⚠ No matter what work is done, the encoding implementation must not change! This could have catastrophic consequences! ⚠

Ideally this should be worked on after !2688 (merged) is merged to avoid conflicts, but there is no hard restriction either