CountedMap with an enforced MaxValue

[kianenigma]

First, the current storage maps have a MaxValues that is not very well documented, and counter intuitively, it is not an enforced in any way. This MaxValues is only used in storage_info related functions.

This needs to be fixed, as a part of paritytech/substrate#13987.

Then, we could consider creating a CountedMap, possibly called BoundedMap that actually has a maximum counter enforced. This is in itself not so exciting, as once the map is filled, it is pretty useless. In other words, it is not sybil resistance. The solution is to expose an API through which an economic game can be created to allow users to kick someone out of the map and replace themselves.

fn insert(key: Self::Key, value: Self::Value) {
  // normal insert, if the map is full, this will fail
}


fn try_insert(key: Self::Key, value: Self::Value, eject: Self::Key) {
  // insert `value` under `key`, and if the map is full, try and eject `eject` from the map.
  // this should use a separate `triage`-like method, provided by the user of the map, to 
  // compare if `key` is more worthy of staying in the map or `eject`
}

This should make the creation of maps that are bounded, and allow users to compete in a position in the map based on some amount of deposit more streamlined.