vm: fix macros.add crashing with openArray argument

[zerbina]

Summary

Fix the compiler crashing when passing openArray value to
macros.add.

Details

• handle akOpenArray in vmtypes.elemType (used by the
  mNAddMultiple implementation)
• handle akString in vmtypes.elemType (fixes nothing, but it
  should still be handled)

[saem]

It might make sense to handle akString but that's more for uniformity/not tripping up assert unnecessarily -- not a blocker, can merge without.

[zerbina]

/merge

[github-actions]

Merge requested by: @zerbina

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• split out from implement a new CGIR and C code generator #1625
• elemType is also used for deserializing openArray values, but compile-time evaluation yielding openArray[NimNode] values is such a fringe use case (that also relies on an experimental feature) that I didn't add a test for it