Add per locale Intl memoizer

[Demivan]

This allows multiple bundles with same locale to share Intl cache

[Demivan]

Sorry for pinging. Anything need to be changed for this to be merged?
I can see that this pull requests fixes same issue #254 tries to fix. If that is preferred API I can change this pull request to match #254 API.

[zbraniecki]

@Demivan we're currently focused on finishing the Rust translation of this library and those PRs for JS are a bit on a backburner. I'm sorry for it. Within a month we'll start looking again at them and hope to start addressing all the backlog here.

In the meantime, if you have a suggestion wrt. the WeakMap, feel free to take over the PR :)

[Demivan]

Hi.
Any chance this gets merged before next release? Anything need to be changed? I have rebased this PR and fixed conflicts.
Is there planned date for next release?

[eemeli]

Sorry for the delay; finally got around to looking at this. And it's good! I think it might make sense to eventually separate the memoiser even further form the bundle, as I at least find it a little surprising how the class, locale arg, and options arg are now all handled in very different parts of the API. But that'd be a larger change than this, which makes sense just by itself.