We should have an `if`/`else` construct

[apoelstra]

When we're matching on a single bit it would be nicer to write this as an if chain.

[apoelstra]

cc @canndrew in case you have opinions about what this should look like

We discussed this in person and I think that even in a dependently-typed SimplicityHL we want if as sugar for a "match where the proofs are unused"

[uncomputable]

There is a Boolean match expression. You could have if () {} else {} as sugar for match () { true => {}, false => {}}.

Or do you mean an if statement without else branch? In that case, there would need to be a default Simplicity expression for the else branch.

[apoelstra]

Yeah, I mean sugar for match ().