consolidate key/value types; consistent interface arguments

Signed-off-by: Jason Volk <jason@zemos.net>

src/database/mod.rs

@@ -6,6 +6,7 @@ mod iter;
 mod map;
 pub mod maps;
 mod opts;
+mod slice;
 mod util;
 mod watchers;
 
@@ -17,6 +18,7 @@ pub(crate) use engine::Engine;
 pub use handle::Handle;
 pub use iter::Iter;
 pub use map::Map;
+pub use slice::{Key, KeyVal, OwnedKey, OwnedKeyVal, OwnedVal, Val};
 pub(crate) use util::{or_else, result};
 
 conduit::mod_ctor! {}