`merge` does not support blob-maps

[helins]

Complains that a blob-map cannot be cast to a hash-map:

(merge (blob-map) (blob-map))

[mikera]

Not trivial to implement efficiently, as it requires special support from the data structures. Can of course do in a simple way with repeated assocs.

Currently we only support regular hash maps for merge, though BlobMap support might make sense in the future.

[helins]

I guess it should be somewhat polymorphic?

[mikera]

Yes I think this support makes sense.

[helins]

Might be best to implement in terms of a repeated assoc so that the behavior is in, then optimize later.