Map restructure

[LindsayWray]

Map Restructure

Currently MAP is internally stored as a struct containing two lists -> STRUCT{ [keys], [values] }
This restructure stores MAP as a list of structs -> LIST[ {key, val}, {key, val}, …] and allows map to be treated internally a list and therefore re-uses a lot of the list functionalities.

This is now in line with the Arrow structure:

Map data is nested data where each value is a variable number of key-item pairs. Its physical representation is the same as a list of {key, item} structs

The memory usage has diminished since now only one list_entry_t struct per map is needed instead of two.

Previous state (Two list_entry_t structs were required for the keys and values separately):

This PR (One list_entry_t struct for the key/value structs):

This alteration also impacted other map functions, namely map_from_entries. Since the input is already parsed as a list of key/value pair structs, 
this memory could be referenced as a whole instead of moving each value into a new block of memory. This does however change the former functionality where NULL elements are (silently) skipped instead of throwing an error when performing this query on multiple rows. See #3600, I’ve also discussed this with @Tishj. To accommodate this functionality, it would not be possible to re-use the memory, since it impacts the lengths and offsets. Or perhaps use a selection vector

So in this PR I’ve opted for referencing the allocated memory and throwing an error when encountering a NULL value (keys can not be NULL)

[Tishj]

Sounds like a good change!

Regarding:

So in this PR I’ve opted for moving the memory and throwing an error when encountering a NULL value (keys can not be NULL)

Do you then also verify that keys don't contain duplicates?

[LindsayWray]

Sounds like a good change!

Regarding:

So in this PR I’ve opted for moving the memory and throwing an error when encountering a NULL value (keys can not be NULL)

Do you then also verify that keys don't contain duplicates?

Yes! It goes through the MapConversionVerify function, that makes all those checks and returns the aproppiate error

[Mytherin]

Thanks for the PR! Great rework. Some minor comments, otherwise this looks good to me: