Simplify API

[eemeli]

This does a few things to simplify the proposed API, largely following recommendations from @sffc following last week's ECMA-402 presentation:

1. Each Intl.MessageFormat instance is now built around a single message resource, at the cost of removing the ability for message references to refer outside their source resource.
2. The Intl.MessageResource primordial is dropped.
3. The format() method is dropped; its effects are similarly available as getMessage()?.toString()

[sffc]

If you make this change, make sure it continues to align with the model of MFv2. My feedback from last week did not consider the full depth of the problem space.

[eemeli]

Absolutely! This proposed change reflects corresponding changes in the ez-spec proposal for the Unicode MF2 spec, which was the only one of the current proposals that included cross-resource message references. That change in turn follows on from discussions earlier this week in the MFWG that covered the use cases for message references in detail, and arrived at a general sense that at least for non-local message references, their use cases may also be well served by user-definable custom functions.

This does enhance the importance of e.g. Intl.MessageFormat instances being memoizable, which actually turns out to be much easier with this slightly more limited API. I filed issue #6 here in part to work further towards that.

[dminor]

lgtm!