the `blockdata` directory is code organisation thing, all the
types/modules are re-exported from other places. In preparation for, and
to make easier, the `primitives` crate smashing work - remove all
explicit usage of `blockdata`.
Note that the few instances remain as they seem required e.g.,
`pub(in crate::blockdata::script)`
Refactor only, no logic changes.
In order that we can safely change/maintain de/serialization code we
need to have regression tests with hard coded serializations for each
type that implements serde.
It is enough to test a single serde data format, use JSON for `opcodes`
and bincode for other types.
Do regression testing in a newly added `tests` module.
Tobin C. Harding