Factor out IDL-to-ECMAScript type conversion

[TimothyGu]

We currently have an incomplete version of the actual algorithms:

webidl2js/lib/constructs/operation.js

Lines 88 to 98 in bf9954d

	if (overloads.every(overload => conversions[overload.operation.idlType.idlType])) {
	str += `
	return ${callOn}.${implFunc}(${argsSpread});
	};
	`;
	} else {
	str += `
	return utils.tryWrapperForImpl(${callOn}.${implFunc}(${argsSpread}));
	};
	`;
	}

Would be nice to factor it out, for e.g. callback function support.

[qwtel]

I've noticed that this section was not doing the right thing for Promise, sequence, and record, so I've made some changes: https://github.com/platformparity/webidl2js/compare/master...platformparity:genericwrap

I didn't factor it out though, nor am I aware that there's an actual algorithm defined somewhere (is this part of WebIDL?).