inprocess: give a useful diagnostic for missing Main.inprocess.

* specs/inprocess_spec.yaml (before): Refactor Main object into a
Base object, and set instance fields in `call`.
Add specification for malformed Main diagnostic.
* lib/specl/inprocess.lua (call): Raise an error if `inprocess`
is malformed, before first use.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>