Deprecate the `Script::to_bytes` function in favour of `to_vec` as we
are doing elsewhere.
Note that `ScriptBuf` has `into_bytes` because it does not copy.
Potentially this should be deprecated in favour of `into_vec`?
Note that in regards to the `to_` prefix this naming as valid according
to convention because the `Script` type is borrowed and `to_vec` copies
the underlying bytes.