Add a `LockTime` type to hold the nLockTime `u32` value. Use it in
`Transaction` for `lock_time` instead of a `u32`. Make it public so this
new type can be used by rust-miniscript and other downstream projects.
Add a `PackedLockTime` type that wraps a raw `u32` and derives `Ord`,
this type is for wrapping a consensus lock time value for nesting in
types that would like to derive `Ord`.
0ed78e543b