Optimize 1-of-N thresholds to or()s

https://github.com/rust-bitcoin/rust-miniscript/pull/113 left out a FIXME that we are unsure to fix at all, opening this to clarify if we want it or not.

We now optimize N-of-N thresholds to and(), and i propose to do the same for 1-of-N thresholds (using or()s). After discussing this with sipa (who implemented this in C++) and @sanket1729 the main issue with the latter seems to be the exponential complexity with the number of or()s.

I personally think (user point of view) that if the complexity growth is only applied on the policy compilation it's fine, as we only ever compile once to miniscript per user and then store the Miniscript Descriptor while the Script size is felt onchain For Ever (™️).

Start by reading this issue and pull request #113, then trace the policy compilation path for N-of-N threshold optimization. Determine whether 1-of-N thresholds should compile to or()s, assess the exponential complexity discussed in the issue, and define a decision or implementation scope with appropriate validation.