Modernize the `RPC` class in `src/shared/frame-rpc` that serves as both
the client and server for RPC requests between frames.

This code was originally imported from
https://github.com/substack/frame-rpc but now that we've adopted it, it
makes sense to modernize it to match the conventions and requirements of
the rest of our code.

 - Convert `frame-rpc.js` to modern syntax
 - Update JSDoc documentation for methods and class
 - Convert `RPC` class to a named rather than default export
 - Inline the `apply` method into the `call` method, since only `call`
   was used externally
 - Remove unused option for `methods` constructor argument to be a
   function
 - Rename `src` and `dest` public fields to the more explicit
   `sourceFrame` and `destFrame`