Implement specialized spliterators on {Entry,Key}Set

[rovarga]

We are currently using SpliteratorIterators backed by our custom iterators. This is a bit wasteful, because it allocates an additional object and in case of MutableTrieMap (and hence Mutable{Entry,Key}Set) we end up wrapping the immutable internal structure with a mutable wrapper for each object.
The second part is important, as it is completely unneeded as spliterators cannot modify state, hence we should be able to just traverse an immutable view of the map.

[asad-awadia]

@rovarga are the keys ordered? (Skiplist/tree map)

[rovarga]

@rovarga are the keys ordered? (Skiplist/tree map)

No. A Triemap is akin to a ConcurrentHashMap in this regard.
That having been said, a ConcurrentNavigableMap with O(1) snapshot capability could be built using the approach used for internal organization of TrieMap -- I think :)

[asad-awadia]

@rovarga

approach used for internal organization of TrieMap

What do you mean by this?

[rovarga]

@rovarga

approach used for internal organization of TrieMap

What do you mean by this?

I mean an equivalent of a TreeMap could be done via #87 .