map: general-purpose map data structures

You should create a map with the `new` function in each submodule.
Then, you may use functions defined in this module to manipulate the map.

Note that maps own keys but not values. Do not change the content of the
[]u8 keys when it's already handled by the map; they are expected to stay
consistent and their lifetime is automatically managed along with the map.
However, you must manage the lifetimes of all *opaque values yourself.