We just re-named `Timestamp` to `BlockTime`. We have a `units::block`
module but it currently holds abstractions (`BlockHeight` and
`BlockInterval`) that are not onchain abstractions and therefore
somewhat different from the `BlockTime`. Instead of making `block` a
block 'utils' module instead re-name the `timestamp` module to `time`.
We just added a `Timestamp` type without knowing that there was a push
by OpenTimestamps to also create a timestamp and that our new type may
lead to confusion. Our timestamp is explicitly for the `time` field in a
block so we can call it `BlockTime`. This name change makes the module
name stale but we will change that in a following patch to ease review.
Bitcoin block headers have a timestamp. Currently we are using a
`u32`. while this functions correctly it gives the compiler no chance
to enforce type safety.
Add a `Timestamp` newtype that is a thin wrapper around a `u32`.
Document it and test the API surface in `api.rs`.