Check compile-time dependencies on macros

[josevalim]

• All modules and calls in macros should be a compile-time dep
• defguard and defguardp should not add compile-time deps
• If var in @attr is used and @attr expands to a list of aliases, we should emit a warning instruction users to use defguard and remove the compile-time deps

[fmterrorf]

I'd like to help out on this issue, but I'm not sure where to start. Just wondering if you can give some advice on where to look. Thanks

[josevalim]

Sorry @fmterrorf, your comment fell through the cracks (I had - still have - an arm injury which did not allow me to stay on top of issues as usual). I will tackle this issue now but if you pick something else I may be in a better position to help.