When creating the ParseIntError in `hex_u32` I (Tobin) just cargo cult
programmed the generic stuff without thinking.
- The `is_signed` field is used to denote whether we were attempting to
parse a signed or unsigned integer, it should be `false`.
- The `bits` field should be directly set to 32.
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.
Fmt Bot
Tobin C. Harding