Add digestable witnesses of binary codecs

[craigfe]

Currently, there is no clean way to test that a refactoring of represented types does not change the binary encoding of those types. This requires us to be very cautious when changing types that might effect production systems – e.g. Tezos – and is particularly problematic when using ppx_repr, where the type representation does not show up in diffs.

To fix this problem, we should expose something like a "witness" of a particular binary codec that can be checked for equality:

type shape [@@deriving repr]
val of_type : 'a ty -> shape

with the idea being that equality of shapes implies equivalence of derived binary codecs. User libraries could then write expect tests of the digests of these shapes, providing some assurance of codec compatibility across revisions.