LinearMap entry API

[Ten0]

Currently to insert in the LinearMap and get a mutable reference to the corresponding value (map.entry(key).or_insert_with(...)), it's necessary to .insert() then .get().unwrap(), because there is no entry API or assimilated.

This implies:

• Cloning the key, in order to be able to call get()
• Iterating on the map twice: once to realize the entry is not there, and once to get the inserted entry

There was already an issue for entry APIs for maps: #103, but that was closed after only implementing it for only one of the map types: #289 - I would tend to believe that this was not intentional.