Add initial version of store interface

The interface enables users to both create and remove entries. It also
features methods for retrieval of both unlocked and lockes entries and a
method for writing back the latter one only.

libimagstore/src/store.rs

@@ -0,0 +1,17 @@
+use std::result::Result as RResult;
+use std::string;
+
+pub use entry::Entry;
+pub use error::StoreError;
+
+pub type Result<T> = RResult<T, StoreError>;
+pub type LockedEntry = SingleUseLock<Entry>;
+
+pub trait Store {
+    fn create(&self, entry : Entry) -> Result<()>;
+    fn retrieve(&self, id : string) -> Result<LockedEntry>;
+    fn retrieve_copy(&self, id : string) -> Result<Entry>;
+    fn update(&self, LockedEntry) -> Result<()>;
+    fn delete(&self, id : string) -> Result<()>;
+}
+