Support recursive importing of modules

[disnet]

Adds support for multiple levels of importing.

// a.js
export function a(x) { ... }

// b.js
import { a } from 'a.js';
export function b(x) { return a(...) }

// c.js
import { b } from 'b.js' for syntax;
syntax m = ctx => {
  b(...);
  ...
}