Fixing constructors for structs in pseudocode

[kevinlewi]

Instead of the using the syntax

Create Envelope envelope with (envelope_nonce, auth_tag)

in pseudocode, we now say:

envelope = Envelope {
  envelope_nonce,
  auth_tag
}

This is done for all of the instantiated structs in the protocol.

Also, we are renaming the nonce field of an Envelope to be called envelope_nonce, as this will match the protocol and test vectors.