Introduce notion of user code

[rekmarks]

Ref: #29

The job of the kernel is to facilitate fearless cooperation between mutually suspicious programs. One of these programs is the kernel itself, and the application it's running in (i.e. MetaMask). The others are third-party programs executed by the kernel. These programs consist of user code, conceptually running in "user space", as distinct from the kernel itself, which runs in "kernel space". We need to establish the boundaries of user space within the kernel, and provide the means to execute user code in vats at runtime.