fix(network): drop the [2,…] message tag wrapper from the rejection reason

[michalrus]

The bug

Currently, the rejection reason is always prefixed with 8202, because the entire d.input() is copied there. And d.input() always contains the whole input.

An (incorrect) example of a FeeTooSmallUTxO error:

82                                      # array(2)
   02                                   # unsigned(2)
   81                                   # array(1)
      82                                # array(2)
         06                             # unsigned(6)
         82                             # array(2)
            82                          # array(2)
               01                       # unsigned(1)
               82                       # array(2)
                  00                    # unsigned(0)
                  83                    # array(3)
                     06                 # unsigned(6)
                     1B 000000025050D4DD # unsigned(9937409245)
                     1A 003D0E39        # unsigned(4001337)
            82                          # array(2)
               01                       # unsigned(1)
               82                       # array(2)
                  00                    # unsigned(0)
                  83                    # array(3)
                     05                 # unsigned(5)
                     1A 0002851D        # unsigned(165149)
                     19 0539            # unsigned(1337)

This is not correct, because the first 2 bytes representing [2, …] are just a tag for Message::RejectTx, which we already give to the user, so the information is currently encoded twice.

The above example should have started on line 3 (3rd byte).

The fix

We need to:

• offset d.input() in RejectReason by the current d.position(),

• I've also moved d.set_position(_) – it's the cleanest to have it in the decoder for RejectReason, so that it behaves like other well-formed decoders, i.e. increments the current .pos.