Adds constructors to allow directly creating locktimes from time or
block counts; adds a flooring constructor to Time to match the ceiling
one; adds an explicit constructor to Height since the From<u16> was not
very discoverable.
Currently we are deriving the serde traits for the `absolute::{Height,
Time}` types, this is incorrect because we maintain an invariant on
the inner `u32` of both types that it is above or below the threshold.
Manually implement the serde traits and pass the deserialized `u32` to
`from_consensus` to maintain the invariant.
Close: #2559
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.
Andrew Poelstra
Tobin C. Harding