* Change option syntax to <name>=<value>, with mandatory "=". This is
still not great, but manageable.
* Remove the Session::require_vspace() function, whose named overload
was misused (to check for the existence of a named space), and the
associated exceptions. Explicit error flow is better error flow.
* Change the autocompletion system to generate all options in a vector
of strings and then iterate, rather than using generators.
* Add completion for symbols.
Lephenixnoir