feat(kernel): Setup direct MessagePort connections between vats and kernel

[grypez]

PoC: #84

We can have the kernel and its vats communicate directly over a message port, without proxying through the offscreen glue code.

Regarding kernel <--> vat communications, the offscreen document is responsible for:

• creating new vat workers
• providing an introduction between the kernel and the vat
• destroying vat workers

All other interactions between the kernel and its vats can take place directly over message ports.

Communication Diagram (Draft)

Suggested Implementation

Since it is possible to send a MessagePort over a MessagePort, we could have the kernel initialize a MessageChannel, send one end of the channel to the Glue code as part of the requests to make a new vat, immediately add the Vat abstraction to its records, and carry on as usual. As a bonus, this implementation does not require first getting the kernel into its own iframe (#57).

[grypez]

Handled in #131.