LNPBP-5: Unset value of tx position and input/output index fields

hello!
the two fields 32-47 and 49-63 use 0 to encode no value. This has a couple of drawbacks:

• the coinbase transaction cannot be represented (as @dr-orlovsky specified);
• the input/output index values need an aritmetic operation (-1) before be used as index.

I think that the encoding would be simpler if N is encoded as N, and INT_MAX(bits) is used to encode UNSET.

I would define the constants

• UNSET_TX_INDEX = INT_MAX(16) = 0xffff
• UNSET_IO_INDEX = INT_MAX(15) = 0x7fff

I would redefine the entity Block; On-chain

First bit set to 0, bits 32-47 set to UNSET_TX_INDEX, bit 48 set to 0, bits 49-63 set to UNSET_IO_INDEX

and the entity Transaction; On-chain to

First bit set to 0, bit 48 set to 0, bits 49-63 set to UNSET_IO_INDEX

Start by locating the Block; On-chain and Transaction; On-chain entity definitions and their uses in the repository. Verify how bits 32–47 and 49–63 currently encode unset values, assess the coinbase and index implications, and update the specification and affected consumers so the proposed sentinel values are consistent everywhere.