External memoization

[zbraniecki]

In #217 I'm proposing an FTLResource class to store parsed FTL resources.

Another vector of cleaning up the role of MessageContext as a immutable container is to avoid duplication of intl memoization between multiple contexts.

Currently, if we create one context for ["res1.ftl", "res2.ftl"] and one context for ["res2.ftl", "res3.ftl"], we'll get two storages of intl objects. In the scenario Firefox operates in, that redundancy is not necessary, and adds a cost to dynamic recreation of contexts when the list of resources changes.

I'd like to suggest an optional external memoization model which could be shared across MessageContext instances:

let memoizer = new IntlMemoizer();
let ctx = new MessageContext(locales, [res1, res2], { memoizer });
ctx = new MessageContext(locales, [res1, res2, res3], { memoizer });

In this model, invalidating ctx1 when res3.ftl is added is very cheap.

In the example above I even folded addMessages/loadResource into the constructor, but if you prefer to keep it as a separate method, it would work the same way.

@stasm, @Pike - thoughts?

[stasm]

#254 has the proposed implementation for this.