Ticket 471: Add `bytevector` procedure

We don't have a procedure bytevector analogous to list, string, and vector. This is a proposal to add one with the obvious syntax and semantics.

When we voted bytevectors into the language in the second ballot, the version of BlobAPI said: "Because there is no preferred way to interpret the data in a blob, there is no blob function analogous to list or vector and no second argument to make-blob." However, we overrode the second part on the third ballot, adding a second argument to make-bytevector. I propose that we now override the first part as well.