User-Friendly Data Slicing

[sffc]

This is the meta-issue to track tasks relating to user-friendly slicing of ICU4X locale data.

• [x] #948 -- the primary mechanism for us to perform data slicing
  • [x] #1106 -- prerequisite
• [ ] ~#1108 (Stretch for Q4) -- a secondary option that may be more versatile~ (deprioritized)
• [x] #1107 -- required for loading data in smaller chunks
• [x] #718 -- required for user-friendly toolchain automation
  • [x] #1105 -- prerequisite
  • [x] #455 -- follow-up
• [ ] #664

More sub-tasks may be added as work progresses.