- All public Swift-defined structs except CdsiLookupResponse,
which wraps LookupResponseEntryList, which can *probably* be made
Sendable but I didn't spend time on it.
- All public Swift-defined enums except those being used purely as
namespaces.
- All zkgroup types, since they are immutable and can be serialized
and deserialized to send them anyway
- ServiceId and its subclasses, an immutable class hierarchy
- ProtocolAddress, PinHash, SenderCertificate and ServerCertificate,
and all public and private Key types, immutable wrappers around
immutable Rust objects
More of our wrapper types could be made Sendable as well if there's a
need to. See CODING_GUIDELINES for more info.
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.
