feat(system-module): rewrite

> Following #209 #213 and 136bddab33686133f3c05bedd2773d0877a43a22
>
> Here is my rewrite of the --module system transform.

#### This PR still fails 3 tests !
@guybedford @sebmck > Can you clarify some behave for me ?

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- [x] @sebmck > When transforming a exported constant in [`test/fixtures/transformation/es6-modules-system/exports-variable/actual.js#L6`](https://github.com/6to5/6to5/blob/v1.15.0/test/fixtures/transformation/.es6-modules-system/exports-variable/actual.js#L6):

```js
export const foo6 = 3;
// is transformed to
const foo6 = 3;
```

**Is it normal ?**

> For me,
> The "normal" transformation through
>
```js
SystemFormatter.prototype.import =
  SystemFormatter.prototype.export = function (node, nodes) {
    nodes.push(node);
  };
```
> must change
```js
export const foo6 = 3;
// to
export var foo6 = 3;
```
> before the `SystemFormatter.prototype.transform` occur.

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- [ ] @guybedford > When transforming exported default class in [`test/fixtures/transformation/es6-modules-system/exports-default/actual.js#L6`](https://github.com/6to5/6to5/blob/v1.15.0/test/fixtures/transformation/.es6-modules-system/exports-default/actual.js#L6):

```js
export default class {}

// You expect :

System.register("actual", [], function (_export) {
  "use strict";

  var __moduleName = "actual";

  var _anonymous;
  return {
    setters: [],
    execute: function () {
      _export("default", _anonymous = function _anonymous() {});
    }
  };
});

// However I'm generating :

System.register("actual", [], function (_export) {
  "use strict";

  var __moduleName = "actual";

  function _anonymous() {}

  return {
    setters: [],
    execute: function () {
      _export("default", _anonymous);
    }
  };
});

```

**Is it still bad ?**

> BTW Adding method in the class generates
```js
export default class Bar{ get foo(){} }
>
// to
>
System.register("actual", [], function (_export) {
  "use strict";
>
  var __moduleName = "actual";
>
  var _classProps = function (child, staticProps, instanceProps) {
    if (staticProps) Object.defineProperties(child, staticProps);
    if (instanceProps) Object.defineProperties(child.prototype, instanceProps);
  };
>
  return {
    setters: [],
    execute: function () {
      _export("default", (function () {
        var Bar = function Bar() {};
>
        _classProps(Bar, null, {
          foo: {
            get: function () {}
          }
        });
>
        return Bar;
      })());
    }
  };
});
```
> Here I don't think `Bar` will be available in the rest of the code... **Does it's matter ?**

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- [ ] @guybedford >  When transforming exported a named function in [`test/fixtures/transformation/es6-modules-system/hoist-function-exports/actual.js#L3`](https://github.com/6to5/6to5/blob/v1.15.0/test/fixtures/transformation/.es6-modules-system/hoist-function-exports/actual.js#L3):


```js
export function nextOdd(n) {
  return isEven(n) ? n + 1 : n + 2;
}

// You expect :

System.register("actual", ["./evens"], function (_export) {
  "use strict";

  var __moduleName = "actual";
  var isEven;

  function nextOdd(n) {
    return isEven(n) ? n + 1 : n + 2;
  }
  _export("nextOdd", nextOdd);

  return {
    setters: [function (m) {
      isEven = m.isEven;
    }],
    execute: function () {}
  };
});

// However I'm generating :

System.register("actual", ["./evens"], function (_export) {
  "use strict";

  var __moduleName = "actual";
  var isEven;

  function nextOdd(n) {
    return isEven(n) ? n + 1 : n + 2;
  }


  return {
    setters: [function (m) {
      isEven = m.isEven;
    }],
    execute: function () {
       _export("nextOdd", nextOdd);
    }
  };
});
```

**What is this `_export` here ?**

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<h3 align=center>Thanks</h3>

<p align=right>@douglasduteil.</p>

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