Phase 4 tracking issue

[bvisness]

The entry requirements for phase 4 are:

• Two or more Web VMs have implemented the feature and pass the test suite (where applicable).
• At least one toolchain has implemented the feature (where applicable).
• The spec document has been fully updated in the forked repo.
• The reference interpreter has been fully updated in the forked repo and passes the test suite.
• The Community Group has reached consensus in support of the feature and consensus that its specification is complete.

Blocking issues:

• Where to place the externtype in the text format? #11
• Is deduplicating externtypes worth the complexity? #13
• Zero-length import groups? #15