Invert miniscript dependency

[uncomputable]

We remove elements-miniscript from rust-simplicity so that the former can depend on the latter. It will be much easier to implement Simplicity descriptors in elements-miniscript where there exists most of the required infrastructure. Doing this in rust-simplicity would lead to a lot of copy and pasting.

We decided to create a new abstraction layer for Simplicity policy (keys, satisfiers, ...) to keep all the messy details about constructing Simplicity programs in rust-simplicity. elements-miniscript can simply implement these traits for the existing analogues (MiniscriptKey, Satisfier, ...) to make use of them.

One drawback of this PR is that Policy no longer implements FromStr. We would have to copy a lot of code from elements-miniscript to adapt the approach from master, and we don't use FromStr outside of tests. elements-miniscript can use its Tree trait to parse Policy (see commit message).

[apoelstra]

I think it's ok that we can't use FromStr directly on Policy. Policy shouldn't occur outside of descriptors anyway, and elements-miniscript can do string-parsing there using its own FromTree trait (which it can impl on Policy), as you observe.