Bug 914414 - Refactor l10n.js, take 2

See #12050 for the first version, with description.  This is a new version of the refactor, written completely from scratch with utmost attention to performance and memory consumption. The source is located in [stasm/l20n.js/tree/placebo](https://github.com/stasm/l20n.js/tree/placebo); `shared/js/l10n.js` in this pull request is a dist version built from source.

[Bug 914414](https://bugzil.la/914414) has more details.