Make the examples runnable by cargo.

[Fraser999]

This allows us to run the examples by calling e.g.
'cargo run --example hello_world'

It also means the examples get compiled when calling 'cargo test'.

[seladb]

When I wrote these examples I debated between writing them as separate applications (with different TOMLs) and writing them as rs files that are being built together with the library.

I chose the first option because I think it's cleaner and it also gives the user a real example app that uses the library

[seladb]

The subsequent PRs rely on these changes so let's sort this one out first and then I'll review the other ones

[Fraser999]

I updated this to retain your preferred project structure, but still to allow the expected cargo run --example ... to work.