Syntax for importing macros from external modules into scope

Created by: andreypopp

Heavy WIP. Should be pulled on top of #227, just the latest commit is interesting actually.

Adds "import macros from ..." primitive macro which mimics a part of ES6 module syntax and brings every exported macro from an external module into scope of the current module.

import macros from "./id.sjs"

var x = id 42

At first, I wanted to implement just standard ES6 module syntax but the realized that there's no way to differentiate between runtime module imports and compile-time module imports.

Implemented as a primitive macro.