Separate out "public API" utils from private ones

[domenic]

In particular, jsdom at least uses:

• wrapperForImpl
• implForWrapper
• tryWrapperForImpl
• tryImplForWrapper

But it imports these from the same generated utils.js file that also exports a bunch of other stuff, like mixin, implSymbol, iterInteralSymbol, etc.

Instead we should have something like webidl2js-utils.js (public API, those above) and webidl2js-wrapper-helpers.js (private API, the rest). Of course, webidl2js-wrapper-helpers.js can re-export the contents of webidl2js-utils.js so that the wrappers only have to import one file.

Then we can document the public ones.