(Feature suggestion) SortedMultiSet

I think that this package is missing a SortedMultiSet type. As an example, let's say we want to collect a list of persons, sorted by age, as stored in an ages dictionary:

ages = Dict(:bob => 1, :sam => 2, :joe => 1)

and perform efficient insertions/deletions and searching on this list. My first reflex is using a SortedSet for this; however: SortedSet(keys(ages), Base.Order.By(x->ages[x])) merges the keys :bob: and :joe. (*)

There are two ways to work around this limitation right now:

1. SortedSet((age, person) for (person, age) in ages);
2. SortedMultiDict(age => person for (person, age) in ages).

However, both of them are somewhat memory-inefficient (they store information equivalent to a full copy of the ages database, as well as quite cumbersome to use (neither implementation allows a simple delete!(set, :bob) call). The second inconvenient may be fixed by writing a simple wrapper over either type (possibly the SortedMultiDict, which is somewhat more efficient), but the first one cannot be fixed except by writing a new structure.

(Also, keep in mind that the example I gave here is of course much simplified. In particular, the comparison values are numbers, and thus quite compact in memory. There could probably be some use cases where the comparison keys are big data structures, hence the fact that we don't want to copy all of them).

(*) in my view this is simply a bad design choice in the definition of SortedSet, because only equal values should be merged, i.e. a set should be allowed to be sorted according to a preorder, but it is likely much too late to change this.

Start by reviewing the existing SortedSet and SortedMultiDict implementations and their APIs. Define how a SortedMultiSet should handle duplicate comparison keys, efficient insertion/deletion/search, and deletion by value; the issue does not identify files or tests, so completion criteria would need maintainer agreement.