erts: Optimize maps:merge

to be better at reusing entire hashmap sub-trees.

Sub-tree reuse is detected in three cases:

1. The sub-tree top node does not exist at all in the other map.
   Already implemented before this commit.

2. The exact same sub-tree exist in both maps.
   Must calculate nr of keys in tree to get total size right.

3. We detect that a sub-tree only contains stuff from one of the maps.

There is still one case we don't detect. If A and B leafs have equal
keys we could also compare the values. If values are equal, further
node reuse could propagate up toward the root (by 'mix'==0).
The downside would be potentially expensive value comparisons.

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