The version 1.63 satisfies our requirements for MSRV and provides
significant benefits so this commit bumps it. This commit also starts
using some advantages of the new MSRV, namely namespaced features, weak
dependencies and the ability to use trait bounds in `const` context.
This however does not yet migrade the `rand-std` feature because that
requires a release of `secp256k1` with the same kind of change - bumping
MSRV to 1.63 and removing `rand-std` in favor of weak dependency.
The `absolute` and `relative` locktimes as well as the `Sequence` are
all primitive bitcoin types.
Move the `Sequence`, and `locktime` stuff over to `primitives`.
There is nothing surprising here, the consensus encoding stuff stays in
`bitcoin` and we re-export everything from `blockdata`.