Collection and its subclasses are now just new-types for tuple removing
the previously awkward `dependencies` field exposed for the engine. The
engine is updated to handle these Collections as Iterable Python
objects when attempting to collect their contained values.

Test failures revealed isinstance for types derived from tuple behaves
unexpectedly under Python3.6. Although this can be worked around by
saving off the mro in __init_subclass__ and using that to perform a
custom isinstance check, more investigation indicated two aspects of
the status quo were undesirable:

1. By returning NotImplemented we left equality up to foreign types
to decide.
2. The intended use of Collection subtypes is for the engine, which
treats all types as distinct; yet we would compare subtypes as equal.

As such implement exact type match based equality and document this.