Use dyn Trait for working with databse

It improves compile time in `--release` mode quite a bit, it doesn't
really slow things down and, conceptually, it seems closer to what we
want the physical architecture to look like (we don't want to
monomorphise EVERYTHING in a single leaf crate).

crates/ra_db/src/lib.rs

@@ -32,6 +32,10 @@ macro_rules! impl_intern_key {
     };
 }
 
+pub trait Upcast<T: ?Sized> {
+    fn upcast(&self) -> &T;
+}
+
 pub trait CheckCanceled {
     /// Aborts current query if there are pending changes.
     ///