New server middleware `serve()` (while deprecating `telefunc()`)

[brillout]

Same as telefunc() but returns the whole context object. See:

• Caching #180

Also, it's a better name.

We don't need it for now (because onAfter() is enough for the permission use case).

Let's postpone implementing this until we stumble upon a use case for it. (But let's eventually implement this as it's a better name.)

See also:

• An easier way to centralise permissions check for telefunctions #186
• New hook onBefore() (for permissions) #189
• New hook onAfter() (for caching) #190