Move the following unit types to the new `units` crate:
- `locktime::absolute::{Height, Time}`
- `locktime::relative::{Height, Time}`
- `FeeRate`
- `Weight`
Also move the `parse` module as well as constants as required.
Do minimal changes to get things building:
- Feature gate on "alloc" as needed.
- Remove rustdocs that use `bitcoin` types.
- Re-export units types so this is a non-breaking change.
- Fix import paths.
Tobin C. Harding