ES Module Support with import/export

[levjj]

Adding support for import and export statements. This also includes importing and exporting of annotations such as pre-, postconditions and invariants.

Example:

// File a.js
export function f(x) {
  requires(x >= 0);
}

// File b.js
import {f} from './a.js';
f(23); // Verified precondition
f(-1); // Precondition violated