We would like to move the `Script` type to `primitives` without moving
any key stuff, including pubkey hashes. We may later, before releasing
`primitives`, move the `WPubkeyHash` at which time this patch can be
reverted or re-implemented on `ScriptBuf`.
The `ScriptBuf::p2wpkh_script_code` function does not take `self` as a
parameter but it does return `Self` - this can trivially be made into a
standalone function.
Make `ScriptBuf::p2wpkh_script_code` a standalone function.
feef34fdea