And consolidate the implementations of these two separate checks; now
they both check for a valid session by looking for a sender chain
instead of just *some* current session, in addition to the new check
for an expired unacknowledged session. At the Rust level, this is now
one check named has_usable_sender_chain; at the app levels, the old
names of hasSenderChain (Java) and hasCurrentState (Swift, TypeScript)
have been preserved.
Tests to come in the next commit.
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.
