impr: Flatten externals

[aleksanderkatan]

TO BE RECONSIDERED: we now generate error messages based on AST, this would obfuscate them.

Blocked by #2019

Externals can be flattened to save memory. For example:

const foo = () => {
  'use gpu';
  const a = this.config.multiplier;
  const b = this.config.zero;
  const c = this.config.multiplier;
};

const externals = {
  'this': {
    'config': {
      'multiplier': () => this.config.multiplier,
      'zero': () => this.config.zero,
    },
  }
}

can later on be simplified to

const foo = () => {
  'use gpu';
  const a = d; // only in AST, the function code is left as is
  const b = e;
  const c = d;
};

const externals = {
  'd': () => this.config.multiplier,
  'e': () => this.config.zero,
}