Wrap generic return values

[qwtel]

Fixes wrapping of implementation classes for Promise, sequence, and record return types.

Currently, when a operation returns a promise, array, or object, the generated output will be

return utils.tryWrapperForImpl(${callOn}.${implFunc}(${argsSpread}))

which does not accomplish the goal of wrapping the implementation class for the objects contained inside.

Using promises as an example, the output should instead be:

const res = ${callOn}.${implFunc}(${argsSpread});
return res && res.then ? res.then(utils.tryWrapperForImpl) : res;

[TimothyGu]

Let's make the code stricter. Only check res is null if the return type is nullable, and if it's not null (or the return type is not nullable) just do res.then(utils.tryWrapperForImpl).

if (returnType.nullable) {
  str += `
    if (res === null) {
      return res;
    } else {
  `;
}
str += `return res.then(utils.tryWrapperForImpl);`
if (returnType.nullable) {
  str += "}";
}

Ditto for the two blocks below.

[TimothyGu]

This assumes that the return type of all potential overloads are the same. (That's why this.idls is an array – all overloaded operations with the same name are grouped into the same Operation object.)

[domenic]

Resurrecting this would be helpful for the Streams Standard reference implementation, which has a tee() method that returns an array. Currently I need to manually map the results using wrapperForImpl.