Improve CI speed

[GabrielMajeri]

CI takes a really long time, and in the case of AppVeyor which does not run jobs in parallel it can take several hours.

Ideas

• Use Cargo workspaces, to avoid rebuilding the same crates multiple times

Issues

• For some runners running the examples is disabled, because of the 50 minutes time limit.
• The aobench benches aren't built / tested, and they would also make CI slower.