Add the length prefix when formatting hex strings by way of `LowerHex`
and `UpperHex`.
This looses formatting options because I can't remember right now how
not to - again.
I'm not sure why we do not use consensus encoding currently for encoding
and decoding scripts to/from hex strings. Many tests include hard coded
hex which do not include the length prefix.
- Add a pair of encoding functions to encode/decode to/from hex without
the length prefix.
- Make `to_hex` and `from_hex` expect the length prefix i.e., use
consensus encoding.
This makes the API easier to use because the various encoding APIs can
be use together now eg `consensus::encode_hex` and `ScriptBuf::from_hex`.
Our encoding/decoding support for scripts is currently a bit convoluted.
Add an example that shows how all the APIs go together.
Note this highlights a potential problem. `to_hex_string` is in
`bitcoin` and therefore has access to consensus encoding but does not
include the length prefix. This means that the output string from
`to_hex_string` cannot be parsed with
`consensus::encode::deserialize_hex` - which is surprising and has lead
to difficulty by at least one user trying to debug.
Tobin C. Harding