Return err instead of panic for invalid map keys

[begelundmuller]

It seems DuckDB supports more map key types than Go, causing the assignment out[key] = val to panic. This PR adds a check that map key types are comparable and returns an error if not.

An alternative would be to not scan DuckDB maps into Go map, but instead directly return the list object containing {"key": ..., "value": ...} maps. What do you prefer?

[marcboeker]

Good catch, thanks for the fix! I suggest we keep it as it is at the moment and if there is demand for non comparable map keys we can switch to the list approach.

[begelundmuller]

Sounds good – thanks for the fast review!

[begelundmuller]

Just adding a note that I discovered that the Python driver takes the {"key": ..., "value": ...} approach:

>>> duckdb.sql("SELECT map(['B', 'A'], [map([1, 2], [3,4]), map([1, 2], [3,4])]);").fetchall()
[({'key': ['B', 'A'], 'value': [{'key': [1, 2], 'value': [3, 4]}, {'key': [1, 2], 'value': [3, 4]}]},)]

Might be worth doing at some point to support all cases, even though it has worse ergonomics.

[marcboeker]

This looks so weird to me, but yeah it would make sense to be more flexible. Shouldn't be that complicated to implement though.