Add Map.insert_or_fail

[Dean177]

This function should have the following signature:

val insertNoOverride :  'key 'value 'id Map.t -> key:'key -> 'value -> ( 'key 'value 'id Map.t) 'value Result.t

It should return an Ok containing the newly update map *if key has not already been set.

If it has it should return an Error containing the existing value for the key.

Open to suggestions for the name. Some inspiration:

• (insert / add)NoOverride
• insertOrFail
• insertUnique