Audit for redundancy

There are some methods in Nvelope which have since been added to Linq, making the Nvelope ones redundant. We should go through and take those into account - either deleting the redundant methods (and doing a major version change), or at least deprecating them.

For example:
MapIndex is the same as LINQ's ToDictionary(fn)
Distinct(fn) is the same as LINQ's SelectDistinct(fn)
