Use bitcoin-units 1.0

[apoelstra]

Draft until 1.0 is out; for now we are using rc0.

This introduces several changes that move this crate toward its final 1.0 form:

1. Remove some deprecated methods
2. Return the bitcoin-units Weight type from weight methods on Transaction and Block.
3. Replace ad-hoc serde impls on locktimes and PSETs with standard ones that use the consensus encoding.
4. Replace the Height and Time types in the locktime module with ones from bitcoin-units.

This will improve our compatibility with rust-bitcoin and also improve compatibility with different versions of rust-elements.

[tcharding]

4-1 red lines to green, got to be happy with that.

[apoelstra]

Heh, well, much of it is replacing the serde stuff which I think I will PR separately because I want to get those changes in ASAP.