Allow for shared intl memoizer

[zbraniecki]

At the moment each new FluentBundle gets new IntlLangMemoizer and in result new PluralRules instance.

This has been floated around for a bit in projectfluent/fluent.js#218 and https://bugzilla.mozilla.org/show_bug.cgi?id=1475356

The Rust intl-memoizer has the concept of IntlMemoizer that stores its lang-memoizers internally, we could use it by having L10nRegistry instance have its own instance of IntlMemoizer which returns a reference to IntlLangMemoizer for a given lang that is then used by each FluentBundle.

We'd likely need to add a threadsafe version of that pan-lang memoizer.