Add support for expression vs statement position macros

[natefaubion]

On a few occasions I would have liked to expand to different things when used in a statement position versus an expressions position. One simple use case is macros that expand to expressions but require intermediate bindings. Right now we just have to wrap everything in an IIFE, but that has drawbacks. it would be great if we could drop it if we knew it wasn't necessary (statement position).

[jayphelps]

:thumbsup: would love this.