Add simple state example

[joshtriplett]

I found this example useful in understanding how a tide-based server can
handle and maintain state.

[joshtriplett]

Re-pushed with formatting fixed.

[jbr]

Thanks! I think we'd probably want to encourage people to put the Arc inside of their State (struct State { value: Arc<AtomicU32> } and Server<State> instead of struct State { value: AtomicU32 } and Server<Arc<State>>), since often State will contain types that are already Clone, like database connection pools. Happy to be convinced that this is a better exemplar, though

[joshtriplett]

@jbr Perfectly happy to do it that way instead.

(If you have multiple fields that need Arc, it may make sense to wrap all those fields in a single structure to use only one reference count and one atomic operation, but that structure could still be inside the outermost state structure.)

[jbr]

Thanks!