At this point, the only special behavior of bridge_fn_buffer is to
support multiple return values for the C bridge (a pointer/length
pair), and that doesn't pull its weight. Remove it in favor of a plain
bridge_fn.
This did reveal that Username_Hash was using bridge_fn_buffer and now
produces a fixed-size array, imported into Swift as a tuple, so this
commit also factors out a new helper invokeFnReturningFixedLengthArray.
This is a variant of AuthCredential that carries two UUIDs, intended
to be a user's ACI and PNI. Why? Because when you've been invited to a
group, you may have been invited by your ACI or by your PNI, or by
both, and it's easier for clients to treat all those states the same
by having a credential that covers both identities. The downside is
that it's larger (both the data, obviously, but also the zkgroup proof
of validity, unsurprisingly).
AnyAuthCredentialPresentation gains a 'get_pni_ciphertext' method,
which will return `None` for the existing presentations and
`Some(encrypted_pni)` for the new credential. Having a separate
credential type but a common presentation type makes it easier for the
server to handle all possible credentials uniformly.
Optimize presentation of credentials (AuthCredentialPresentationV2, ProfileKeyCredentialPresentationV2, PniCredentialPresentationV2). Server will accept V1 or V2 presentations. Clients will produce V2.
Various improvements to FFI to support this, and some minor optimizations (in particular "lazy statics" to avoid redundant loading of SystemParams).
