Key- and value-only iteration

[chipshort]

Closes #1498

This completes step 1, the first half of step 2 and step 4 of the corresponding issue.

The rest is tbd in wasmvm.

[webmaster128]

Great start. Now what is missing is a contract using those imports. This would be implementing ExternalStorage::range_keys and ExternalStorage::range_values manually through the C imports

[webmaster128]

Very nice. We just need a CHANGELOG entry here

[webmaster128]

I was thinking about implementing Iterator for the enum directly:

enum ExternalPartialIterator {
    Keys { iterator_id: u32 },
    Values { iterator_id: u32 },
}

However, I don't see any strong benefit and it makes it harder to add fields, so feel free to ignore if not useful.