Support computed properties for instances.

[pvolok]

This is needed before #2952 can be merged.

Lookup rules:

1. If prop is a string literal (either computed or named), we look it up the prototype chain.
2. If class has an indexer, we add it to try_ts_on_failure of LookupT (only if there is no indexer queued yet, we distinguish by the reason RObjectIndexer).

We can't check indexer earlier because some classes like node's Buffer define indexer but has a parent class.

If this PR works, we can get rid of $key and $value props and use dicttype like in ObjT.

cc @samwgoldman

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[STRML]

This unblocks #2952 and fixes #2057 and #2310 after both PRs are merged.