Extend the `database_storage!` macro such that it allows for lifetimes
to be added to the generated struct. This is necessary if the database
itself has lifetime parameters that need to be threaded through to the
queries being invoked.

Extend the `query_group!` macro to allow for lifetimes to be annotated
on the generated structs and traits.

Remove the `'static` bound from the `Query` trait. This allows databases
with lifetime parameters to be used.

Add the `tests/refs.rs` test which creates a simple database that holds
a reference to an input string.

Manually implement `Debug` for query types since the addition of the
phantom data field causes queries to be printed as
`MyQuery(PhantomData)`. The new implementation prints them as `MyQuery`
again.