Add or optimize functions to Hamt

[Enrique Alfonso Casanovas Pedre]

Functions to add:

• - ofSeq
• - ofSeqStructural
• - ofSeqWith
• - filter
• - exists
• - fold and foldBack (how much sense would foldBack make for an unordered collection)
• - forall
• - iter
• - map
• - partition
• - pick and tryPick
• - findKey and tryFindKey (I don't plan to do this)
• - change

Functions to optimize:

• - toSeq group of functions (Move to a separate issue #25)

Potentially more functions