Enforce multiple return statements

[collinc97]

Overview

Currently a function containing multiple return statements will short circuit upon evaluating the first one.
This is incorrect behavior because we need to evaluate the entirety of each function to satisfy R1CS constraints.

Changes

function main(cond: bool) -> u32 {
    if cond {
        return 1u32
    } else {
        return 0u32
    }
}

In the above function there are two return statements whose results depend on a cond input boolean.
We can conditionally select the desired result from the conditional by passing this cond as an indicator to the function main when it is ready to return.
As expected, return arguments will no longer terminate a function early. Instead, they will pass their result along with their indicator to the function main when it is ready to return.