Description:

This library adds basic collections for PHP.

Collections can be seen as more specialized arrays for which certain contracts
are guaranteed.

Supported Collections:
* Sequences
** Keys: numerical, consequentially increasing, no gaps
** Values: anything, duplicates allowed
** Classes: Sequence, SortedSequence
* Maps
** Keys: strings or objects, duplicate keys not allowed
** Values: anything, duplicates allowed
** Classes: Map, ObjectMap (not yet implemented)
* Sets (not yet implemented)
** Keys: not meaningful
** Values: anything, each value must be unique (===)
** Classes: Set

General Characteristics:
* Collections are mutable (new elements may be added, existing elements may be
modified or removed). Specialized immutable versions may be added in the
future though.
* Equality comparison between elements are always performed using the shallow
comparison operator (===).
* Sorting algorithms are unstable, that means the order for equal elements is
undefined (the default, and only PHP behavior).