Add constexpr if

[rw1nkler]

This PR adds support for the constexpr if expression.

Information about the const modifier is stored in the Conditional class and used later during type checking. For constexpr if, separate type variables are assigned to each branch of the condition. At this stage, the result is not directly bound to them. During the TypeInfo generation, the constexpr condition is evaluated, and one branch of the if is selected. Its type is unified, and the entire if is annotated with that type. The bytecode emitter and IR converter have been updated to inline the selected branch, skipping the unchosen one.

Proper tests for this feature are still needed and will be added soon.

One issue not yet handled correctly in this PR is the conditional spawning of procs, for which an example has been added that shows the problem. Currently, all spawns affect the type, which should not happen as only the selected branch of constexpr if should be considered. One possible approach is to enable constexpr evaluator in the PopulateInferenceTableVisitor and skip traversing the branch that was not taken. However, the PopulateInferenceTableVisitor does not have access to the parametric environment, and the condition relying on the parametrics seems to be the most imprtant use-case for using constexpr if when spawning procs.

[richmckeever]

Let's add tests in typecheck_module_v2_test to prove that for example it's OK to have disparate types in the branches of the const if in a function like:

fn f<N: u32>() -> uN[N] {
  const if (N == 1) {
    u1:0
  } else {
    zero!<uN[N]>();
  }
}

[wsipak]

I've applied changes requested in the review.
I've included a test with the function:

fn f<N: u32>() -> uN[N] {
  const if (N == 1) {
    u1:0
  } else {
    zero!<uN[N]>()
  }
}

This previously failed due to wrong TypeInfo object being used for the conditional evaluation - this is now fixed.

I've also included an example with conditional proc spawning. In order to spawn procs in const if, a change was required in the ProcConfigIrConverter which is also present in this PR.

FYI @richmckeever