an important trait of macros: supported constructs

[liufengyun]

As I mentioned in #19, there's a big difference between macros and compilers. One big trait of macros is to make assumptions on the constructs, as evidenced below:

• https://github.com/monadless/monadless#supported-constructs
• https://github.com/scala/async/tree/master/src/test/scala/scala/async/neg