Implement proper structured cloning between Workers

[kevinkassimo]

This is actually #3317, but we need a clearer issue name for tracking purposes.

Structured cloning implementation might be very useful in ways for transferring tasks between Workers for load balancing, through low-cost transfer of ArrayBuffer (or even SharedArrayBuffer). For example, it might allow an HTTP request to move to another vacant thread if it somehow takes heavy CPU time to respond.

[ry]

depends on #3530 (replace libdeno with rusty_v8)

[bartlomieju]

@ry, @piscisaureus and I discussed this issue at length and decided that structured clone is not gonna make it to 1.0.

We need to update docs accordingly as well as postMessage() method to try and discover if passed value can be JSON-stringified (eg. posting Map will give garbage in parent worker). Also postMessage() should throw is second argument is passed (transfer used to transfer typed arrays between workers).