Add functions to test whether or not two variants are "equivalent"

[melissacline]

The vision: create a series of functions to leverage the elements of VRS to ask whether or not two variants are equivalent, given some definition of equivalence. While there are many potential definitions of equivalence (e.g. same protein translation, structural variation that removes the same exons, intronic insertion of overlapping elements, etc), the specific definition could be encoded by the function, with different functions supporting different definitions of equivalence, and the caller choosing which definition is appropriate.

See the previous slides on sets of variation

This is related to Categorical Variation, since equivalence type could be described as variation category.

[reece]

Here's a much older set of slides with some possibly useful ideas:

https://docs.google.com/presentation/d/17jdhX2ZcesuiEu_ZNniLBWsb9wwsez4SLrUNavY4mCc/edit