Add ternary expressions

[MattWindsor91]

There are some known historical bugs based on the conditional execution of atomics. With this in mind, it makes sense to me to introduce support for x ? y : z in the expression generator: it's a nice convenient way for introducing conditionality on integer expressions.

As usual, this'd need a few new legs adding to Fir, some traversal threading-through, expression generator support, abstraction and reification functions, the works.

[MattWindsor91]

We now have ternaries in everything but the expression generator. Using ternaries in a fun way might involve some exciting refactoring of the expression generator, yay.