Use caching to speed up example builds

We used to have caching for this purpose, but found out that it was causing beta and nightly to fail to build. It was removed in https://github.com/acmcarther/cucumber/pull/24 but should be readded (in a functional way) to get reasonable build times again.

It might be possible to manage this by using the new cargo workspaces feature. https://github.com/rust-lang/rfcs/blob/master/text/1525-cargo-workspace.md

The jist is that it lets everything use the same target directory, which is what #24 used to do (but badly).