Enable `1 : n+1` ocap programming

[rekmarks]

Requires #23 #43 #57

Following #43, which we will say enables 1 : 1 ocap programming (between the kernel and a single vat), we need to enable 1 : n+1 ocap programming (between the kernel and multiple vats). This does not necessarily include the ability of vats to message each other; as long as the kernel can manage (boot, restart, shut down) multiple vats and exchange messages with all of them, and we can easily demonstrate this ability, this issue can be considered closed.

[grypez]

I think we will find it useful to complete #55 before this issue.