Bytestructure descriptors need equality

[johnwcowan]

Currently, you can ask what the bytestructure descriptor of a bytestructure is, but there is no way to check if it is the same as (bs:vector 3 uint16). There needs to be an equality function that will establish this.

[TaylanUB]

Difficult. I would urge users to just use singletons for the time being, though I hope I'll come up with a good solution in the future. Recent changes already added a "metadata" field to the descriptor record type; it was added in a rather ad-hoc way with no clear specification of what it should include; I might flesh this out in the future, or change it to be a number of different metadata fields, to allow better introspection, which would allow to check equality among more.