Policy Relative timelocks do not support time based constructions

[sanket1729]

The current Policy enum only supports block based constructions (it's u16 right now). We should also support time based constructions as they are more natural in smart contracts.

[uncomputable]

Ideally policy includes absolute::Locktime and relative::Locktime. Is the latter implemented for rust-elements? rust-bitcoin wants you to generate relative::Locktime through Sequence. Maybe we should write our own enums.

[apoelstra]

bitcoin's relative::locktime can be constructed derectly or through serde or from a sequence value. There are also constants like Height::ZERO you can convert. What other constructors should we have?

[uncomputable]

Should we use bitcoin's structures or elements's? Keeping everything policy-related consistent with Elements might have benefits down the line.

[apoelstra]

We should use elements structures for everything related to transactions. If structures are missing or they don't match bitcoin we should PR to rust-elements to patch it up. (Though we don't want to put too much work into this -- when we reach rust-bitcoin 1.0 we'll just rewrite rust-elements entirely, so doing a lot af major refactor work before then would be wasted.)

[sanket1729]

I think changing the u16 to u32 should be fine for now. And internally converting should be fine for now.
We can port rust-bitcoin stuff later. Though, I think porting the timelocks files should not be that invasive. Everything is in separate modules in rust-bitcoin