Infallible pairs

[uncomputable]

Simfony expressions are translated into Simplicity expressions with a variable source type. This means that pairs can be constructed infallibly. Use PairBuilder to ensure this invariant at compile time and remove unnecessary unwraps.

[apoelstra]

Overall fab865793ee296920b2bd3fcd91befe9c58697e4 looks great. I feel there is a bit more we could do here:

• The name get I think is confusing. It's too generic to be meaningful but not standard. In the stdlib it's usually a form of optional array indexing. We should either use the generic From/Into trait or rename the method to build or both. I would prefer to use From (see next point).
• Having comp take a non-builder is pretty nifty in PairBuilder. But if it took a PB: Into<P> then it could take either a PairBuilder or a P, thereby avoiding a lot of get calls.
• Good catch that comp(unit,*) does not typecheck in general. comp(*,unit) does, but I guess we don't need this.

I think we may have gotten the compiler to the point where every error path is a real error path that can be taken. Would be good to work on a fuzzer to check this.

[uncomputable]

I will implement your suggestions. Unfortunately, impl From<PairBuilder<P>> for P fails the orphan rule and impl Into<P> for PairBuilder<P> fails due to a transitive implementation of Into from the stdlib. I am looking into using AsRef<P>.

[apoelstra]

Frustrating. Ok, AsRef or Borrow might work, or we can just implement a new dummy From-like trait, or we can implement From for the specific P values that we need to work, or we can add from_pair_builder(PairBuilder) -> Self to the CoreExt trait.

[uncomputable]

I renamed PairBuilder::get and refactored PairBuilder::<P>::comp to accept both &P and &PairBuilder<P>.

[apoelstra]

utACK 5d6ffb7a6def31e5f1e0d3220ad680ed6ca35bcd but I'd like to get #73 in first because it's preventing my local tests from completing.

[uncomputable]

Rebased onto #73