Bug in `push_to_map_of_vec` template

[Niket Agrawal]

push_to_map_of_vec constructs a new fusion map from scratch using make_vector and make_pair, which only contains the single new key-value pair being added. (ref)

This approach:

• Discards all existing entries in the original map.
• Only preserves the current key-value pair being processed.
• Fails to maintain the accumulated state information across multiple operations.

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Here’s how I think this can be fixed:

• Convert the map to a vector representation
• Remove the existing entry for the key (if present)
• Create a new entry with the updated vector (original vector + new element)
• Add the new entry back to the vector
• Convert back to map format

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The fix would maintain the same interface while correcting the fundamental logic error in map manipulation.
I wanted to run it through you and get your feedback @henry-ch before raising a bugfix PR. Please let me know your thoughts on this!