Replace use of ES6 for..of with `punycode.ucs2.decode`

[Arachnid]

(Now confirmed to pass all unit tests!)

This PR replaces the use of for..of with punycode.ucs2.decode an punycode.ucs2.encode, removing the dependency on ES6, and therefore making it more browser-friendly.

[jdkanani]

+1 Please merge as it has no side effects.

[KaiSchwarz-cnic]

@jdkanani until this is done, you may give idna-uts46-hx a try. my fork that already covers this.

[Arachnid]

Any chance of this PR getting some love?