add experimental support for "phaseless" modules

The intent is to support phase-crossing data such as the `exn:fail:syntax'
structure type that is instantiaed by macros and recognized by contexts
that use `eval' or `expand'. Phaseless modules are highly constrained,
however, to avoid new cross-phase channels, and a module is inferred to
be phaseless when it fits syntactic constraints.

I've adjusted `racket/kernel' and improved its documentation a little
so that it can be used to implement a phaseless module (which can
import only from other phaseless modules).

This change also adds a `flags' field to the `mod' structure type
from `compiler/zo-structs'.