[js-api] Fix result types in string builtins

[tlively]

The string builtins that return strings should return non-nullable (ref extern) rather than nullable externref. This change reflects current implementation reality in both Chrome and FireFox.

[Ms2ger]

Seems fine. I'd like @sideshowbarker's review of the echidna failures before merging, though

[jakobkummerow]

LGTM.
FWIW, (ref extern) is what the proposal's Overview has always said. Dunno why the final spec ended up diverging from that; I'm guessing that wasn't intentional.

[tlively]

cc @rmahdav

[sideshowbarker]

Seems fine. I'd like @sideshowbarker's review of the echidna failures before merging, though

Our charter has expired, so we no longer have perms to (auto)publish to TR space. I’ve sent a request for an extension of the charter. If/when that gets OKed, the echidna failures should go away.

[eqrion]

Thanks!

[sideshowbarker]

Seems fine. I'd like @sideshowbarker's review of the echidna failures before merging, though

Our charter has expired, so we no longer have perms to (auto)publish to TR space. I’ve sent a request for an extension of the charter. If/when that gets OKed, the echidna failures should go away.

No more echidna failures. We got our charter extension — so it’s back again to working now. All green 🍀

[Ms2ger]

Thanks!