Returns a new modifiable map, initialized with the specified items.
The map is implemented on top of a hash table with weak keys,
which has a couple of trade offs. Specifically a key-value pair could be
discarded at any point if the key is no longer in ordinary use. Other
considerations are comparable with the normal HashMap.
The map is initialized with the items you supply as arguments. This function deals with arguments in exactly the same way as the newMap function, so that information isn't repeated here.
This needs more documentation, as I'm not very familiar with good use cases.