This enables parallelism for streaming AEAD chunked encryption.
The reason we can't do so at the very end of the pipe chain
(e.g., in `readToEnd`) is because requests for increased
buffering (i.e. `desiredSize > 1`) do not propagate backwards,
only requests for backpressure (i.e. `desiredSize <= 0`) do.
When encrypting/signing a stream, this allows you to indicate whether it's a
stream of Strings or Uint8Arrays (using message.fromText or message.fromBinary,
respectively.)
When signing text, this allows you to control whether to create a cleartext
message or a regular armored text message.
When creating a detached signature, it allows you to control whether it's "meant
for" (verifying against) a cleartext message. A cleartext message has trailing
whitespace trimmed before signing. This fixes the case of passing a detached
signature from sign() to encrypt(). Since encrypt() doesn't create a cleartext
message, the signature would be invalid if the text contained lines with
trailing whitespace.
- The caller might not await them, in which case you get an "unhandled rejection"
- The caller might await them but only after reading the data, in which case you
get an unhandled rejection if there's an error before the end of the data
