Specify what Value is in Plinth and review all functions defined over it

[effectfully]

I've created a PR #6590 optimizing unionValue, but it turned out that the new behavior is different when the given Values contain zeroes. But can Values contain zeroes? Which variants Value-handling functions are supposed to maintain? And we need tests ensuring that whatever variants must be preserved are preserved.

There's a good chance that we won't need to work on this issue though as #6585 may help us move the whole problem into UPLC space and handle it there.