Re-export bitcoin crate

Re-export bitcoin crate

For the same reason the `bitcoin` crate is re-exporting
the `secp256k1` crate the `lightning` crate should
re-export the `bitcoin` crate: to ease the burden on
calling code to maintain compatible `bitcoin` versions.

The `lightning` crate makes heavy use of types defined in
(or re-exported by) the `bitcoin` crate. Compilation will
fail if e.g. comparisons or `match` expressions are done with
types from `bitcoin` crate versions with a non-equal minor
version. This forces calling code to depend on a `bitcoin`
crate with a compatible version. This becomes a maintenance
nightmare once two or more crates, that use `bitcoin` types
in their public API, are used in calling code simultaneously.