Define a Variable struct so that frontends don't have to.

Frontends can still use their own types with `ILBuilder` and
`FunctionBuilder`. This just provides a basic `Variable` struct
for frontends that want it.

lib/frontend/src/variable.rs

@@ -0,0 +1,24 @@
+//! A basic `Variable` implementation.
+//!
+//! `ILBuilder`, `FunctionBuilder`, and related types have a `Variable`
+//! type parameter, to allow frontends that identify variables with
+//! their own index types to use them directly. Frontends which don't
+//! can use the `Variable` defined here.
+
+use cretonne::entity::EntityRef;
+use std::u32;
+
+///! An opaque reference to a variable.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct Variable(u32);
+
+impl EntityRef for Variable {
+    fn new(index: usize) -> Self {
+        assert!(index < (u32::MAX as usize));
+        Variable(index as u32)
+    }
+
+    fn index(self) -> usize {
+        self.0 as usize
+    }
+}