moved crates in lib/ to src/, renamed crates, modified some files' text (#660)

moved crates in lib/ to src/, renamed crates, modified some files' text (#660)

cranelift/frontend/src/variable.rs

@@ -0,0 +1,32 @@
+//! A basic `Variable` implementation.
+//!
+//! `FunctionBuilderContext`, `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 core::u32;
+use cranelift_codegen::entity::EntityRef;
+
+///! An opaque reference to a variable.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub struct Variable(u32);
+
+impl Variable {
+    /// Create a new Variable with the given index.
+    pub fn with_u32(index: u32) -> Self {
+        debug_assert!(index < u32::MAX);
+        Variable(index)
+    }
+}
+
+impl EntityRef for Variable {
+    fn new(index: usize) -> Self {
+        debug_assert!(index < (u32::MAX as usize));
+        Variable(index as u32)
+    }
+
+    fn index(self) -> usize {
+        self.0 as usize
+    }
+}