5da506b506 Add additional re-exports (Tobin C. Harding)
Pull request description:
As we do for other types add two new alias' at the crate root of `primitives` and mirror it in `bitcoin`:
- `BlockVersion`
- `TransactionVersion`
ACKs for top commit:
apoelstra:
ACK 5da506b506aa41b88aa7e8ecdffdcc0478ec72b6; successfully ran local tests
Kixunil:
ACK 5da506b506
Tree-SHA512: 5f91e3aae87b1128256b528d20d9aab562bf054131ed8028e0db789e2b487863ad4c89aaca080d0fbcf760dd160815a1843046c5fab0ed1483230fdedc66402e