Rename the entity_ref module to 'entity'.

This is in preparation for moving a number of data structures into
sub-modules of the 'entity' module:

- EntityList
- EntityMap
- SparseMap

lib/cretonne/src/entity/mod.rs

@@ -0,0 +1,53 @@
+//! Densely numbered entity references as mapping keys.
+//!
+//! This module defines an `EntityRef` trait that should be implemented by reference types wrapping
+//! a small integer index.
+//!
+//! Various data structures based on the entity references are defined in sub-modules.
+
+/// A type wrapping a small integer index should implement `EntityRef` so it can be used as the key
+/// of an `EntityMap` or `SparseMap`.
+pub trait EntityRef: Copy + Eq {
+    /// Create a new entity reference from a small integer.
+    /// This should crash if the requested index is not representable.
+    fn new(usize) -> Self;
+
+    /// Get the index that was used to create this entity reference.
+    fn index(self) -> usize;
+}
+
+/// Macro which provides the common implementation of a 32-bit entity reference.
+#[macro_export]
+macro_rules! entity_impl {
+    // Basic traits.
+    ($entity:ident) => {
+        impl $crate::entity::EntityRef for $entity {
+            fn new(index: usize) -> Self {
+                assert!(index < (::std::u32::MAX as usize));
+                $entity(index as u32)
+            }
+
+            fn index(self) -> usize {
+                self.0 as usize
+            }
+        }
+
+        impl $crate::packed_option::ReservedValue for $entity {
+            fn reserved_value() -> $entity {
+                $entity(::std::u32::MAX)
+            }
+        }
+    };
+
+    // Include basic `Display` impl using the given display prefix.
+    // Display an `Ebb` reference as "ebb12".
+    ($entity:ident, $display_prefix:expr) => {
+        entity_impl!($entity);
+
+        impl ::std::fmt::Display for $entity {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+                write!(f, "{}{}", $display_prefix, self.0)
+            }
+        }
+    }
+}