META: better t()-related APIs in locale module

Issue Summary

This is a meta issue to provide better storage and retrieval of data passing through t() as well as work on APIs of t() itself to expose an easy to use unified API to the rest of Drupal.

Sub-issues

#1213510: A modern t()
#1191614: Make t() formatter available as its own function

#322549: t() should work regardless of the bootstrap phase
#746240: Race condition in locale() - duplicates in {locales_source}
#851362: Add hash column to {locales_source} to query faster locale strings
#1206122: Better/consistent location tracking for translatable UI strings

#488496: Implement missing msgctx context in JS translation for feature parity with t()

Parent issue

#1260534: META-META: Make language support awesome in Drupal 8