Fixed export of enums (fixes #17)

[josephg]

This PR adds a rewrite rule check for exported constants, which allows enums to be exported from the wasm-pack bundle.

They appear in the pkg file like this:

export const MyEnum = Object.freeze({ Bar:0,Zot:1, });

And now get exported correctly in the output:

__exports.MyEnum = Object.freeze({
  Bar: 0,
  Zot: 1
});

[catsigma]

thanks for the PR

[josephg]

Thanks for merging!! 😎