The two _concrete_ changes between version 0.4.0 (where we are before
this commit), and version 0.7.0 are:

  1. Variable length messages are now allowed, this comes with a new 8
     byte prefix for the messages.

     * Our implementation was already using a `[]byte` for the
       message/hash, so no extra API changes are needed here.

  2. The serialization for a blank message and a normal message (for
     nonce gen) is now distinct. A single byte is added (either 0 or 1)
     to indicate if a message was passed into nonce generation.

The major change in musig 1.0.0 is that plain public keys are used as
input to key aggregation.

This helps mitigate an issue discovered in musig2 under certain
scenarios:
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-October/021000.html.