* guix/jupyter/environ.scm: Remove.
* guix/jupyter/environment.scm, guix/jupyter/containers.scm: New files.
* tests/environ.scm: Remove.
* Makefile.am (SOURCES): Adjust accordingly.
(SCM_TESTS): Remove tests/environ.scm.
* guix-jupyter-container.scm <top level>: Turn into...
(run-inner-proxy): ... this.  New procedure.
* guix-jupyter-kernel.scm (%inferior): New variable.
(reply-execute-request): Use 'string-tokenize' and
'specifications->manifest'.  Pass the manifest to 'start-container'.
(module-to-import?): New procedure.
(start-container): Rewrite to take a manifest and to evaluate code with
'eval/container*'.  Set the 'GUIX_ENVIRONMENT' and 'JUPYTER_PATH'
environment variables.
* jupyter/kernels.scm (jupyter-kernel-path): Remove "GUIX_PROFILE"
hack.