Inconsistency with mangling of reserved keywords

[glennsl]

This came up when trying to import an es6 module, where the natural way of doing it breaks due to mangling of the reserved keyword default:

external foo': t = "default" [@@bs.module "some-es6-mod"];
let foo = foo';

is compiled into:

var SomeEs6Mod = require("some-es6-mod");
var foo = SomeEs6Mod.$$default;
exports.foo = foo;

This, on the other hand, works:

external foo': Js.t {. default: t } = "some-es6-mod" [@@bs.module];
let foo = foo'##default;

which compiles into:

var Foo$prime = require("some-es6-mod");
var foo = Foo$prime.default;
exports.foo = foo;

As I've been told, the latter case works because in Js.t, anything goes. But these two cases are functionally equivalent, and in the former case the reserved keyword is quoted, not even able to be used as an identifier. This makes the (non-)mangling rules rather counter-intuitive as well as inconsistent.

[bobzhang]

close as a duplication of #1151