Support nested if statements as output environments

[winitzki]

Currently, only a single layer of if statements is supported when analyzing the output environments in the "shrink" operation at compile time.

However, the go macro already gathers complete tree of possible if statements and other output environments. This tree needs to be transformed at compile time into a more readily usable structure, so that emit-once, emit-at-least-once, and emit-maybe are determined for arbitrary nested if statements interspersed with function calls.

[winitzki]

Closed by mistake.