[DRAFT] WIP: Swifty numeric formatting

[milseman]

Work-in-progress providing a Swifty, extensible alternative to printf-style formatting for numerics.

The output is not meant for end-user consumption, use a locale-rich formatter for that. This is meant for machine and programmer use (e.g. log files, textual formats, or anywhere printf is used).

TODO:

• Audit public surface area (do we want protocols, etc?)
• Finish float formatting
• Tests and examples