Refine handling of Defer and Go instructions.

[mlevesquedion]

A recent PR #305 unearthed some issues with how Defer and Go instructions are handled. These instructions are analogous to Call instructions, with some (important) differences in behavior:

• Defer occurs when the defer keyword is used, which adds the call to a stack of calls to execute before the function returns.
• Go starts the call in a new goroutine, meaning from that point on in the program, the instructions in the forked call could be interleaved with subsequent instructions in any order.

Currently, Defer instructions are not handled at all, while Go instructions are handled like regular calls.