Version matching for Clarity, PoX, Stacks 2.1

[kantai]

Among the many planned features of Stacks 2.1 is upgrades for a few different elements of the system, e.g.,

1. A new Clarity release, with new native functions.
2. A new PoX contract, with new functionality, fixes to old functionality

For many of these, we need a way to reference the new version of the element vs. the old version. For PoX, the published contract won't be named pox, but instead something else, e.g., pox-2. For Clarity, the "versioning pragma" has to be able to specify which version of Clarity a new contract should be evaluated with.

These versions could be set to "2.1", such that the new PoX is pox-2-1. Or these versions could be set to "2", because they are the second versions of these new elements. From a technical standpoint, the second option may make more sense, because future Stacks network upgrades may not upgrade these same elements again, such that a Stacks 2.2 release is using, e.g., PoX 2.1. But, having multiple versions for these elements could be confusing (i.e., documentation explaining a function is available only in "Clarity 2" versus "Stacks 2.1").

I'm pretty ambivalent about this -- mostly because I think I have a pretty bad instinct for naming and versioning, so would be happy to go along with something that people felt strongly about. But I do think that this is something we should decide on now, before messaging about future changes becomes ingrained.