We now automatically spawn the kernel found in the environment instead
of running the "inner proxy".  That simplifies code (the inner proxy is
no longer used) and simplifies usage (no need to remember to do ";;guix
kernel" once you have set up the environment.)

* guix-jupyter-kernel.scm (reply-for-environment-kernel): New
procedure.
(create-environment): Build the profile from MANIFEST, and call
'available-kernel-specs-files' in there.  Return an error message if we
didn't find exactly one kernel spec.
(reply-execute-request): Support ";;guix environment NAME" magic.
Remove support for 'magic-run?' and 'magic-kernel?'.
(start-container): Take a profile rather than a manifest.
[spawn]: Use 'available-kernel-specs-files' and 'exec-kernel' rather
than 'run-inner-proxy'.
* guix/jupyter/magic.scm (magic-env?): Adjust accordingly.
* guix-kernel-demo.ipynb: Update.