Fix put_in_map/set_map_key not setting keys properly

[ollien]

In attempting to solve #347, I ran into other problems that all felt related enough to fix at once

1. Both put_in_map and set_map_key would produce an invalid "nested" map when inserting into an empty map (%{%{foo: bar}})
2. Both put_in_map and set_map_key would call the updater, take its updated value, and then return the zipper at the position of the value. This was inconsistent with the non updater case (i.e. inserting a new key would point to the map itself), so I made these consistent.

I recognize #2 is technically a breaking change, but I figured this couldn't have been intentionally inconsistent. If you would prefer a deprecation/new function, let me know.

Fixes #347

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[zachdaniel]

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