inprocess: new `capture` API, for isolating functions from io.

Provide a way to write examples that call functions which would
otherwise use Specl's io streams, by isolation all io for later
inspection.
* specs/inprocess_spec.yaml (capture): Specify behaviour of new
capture API.
* lib/specl/inprocess.lua (capture): Implement it.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
refactor: move shared initialisation into separate functions.

* lib/specl/inprocess.lua (env_init): Shared initialisation.
(capture, call): Use it.

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