From BIP 141:
> A scriptPubKey (or redeemScript as defined in BIP16/P2SH) that
> consists of a 1-byte push opcode (for 0 to 16) followed by a data push
> between 2 and 40 bytes gets a new special meaning. The value of the
> first push is called the "version byte". The following byte vector
> pushed is called the "witness program".
`WitnessVersion` and `WitnessProgram` are scriptPubkey concerns and
scriptPubkey is basically synonymous with address so in one way it makes
sense that these types are in `address` however we are in the process of
overhauling the `Address` (and `AddressInner`) types so lets move the
witness stuff to `script` and put it in individual sub-modules.
This move helps simplify the address error type also.
Note please, there are a bunch of formatting changes in here in the
error type that I cannot explain and could not remove.