Lint fixes (#99)

* Replace a single-character string literal with a character literal.

* Use is_some() instead of comparing with Some(_).

* Add code-quotes around type names in comments.

* Use !...is_empty() instead of len() != 0.

* Tidy up redundant returns.

* Remove redundant .clone() calls.

* Remove unnecessary explicit lifetime parameters.

* Tidy up unnecessary '&'s.

* Add parens to make operator precedence explicit.

* Use debug_assert_eq instead of debug_assert with ==.

* Replace a &Vec argument with a &[...].

* Replace `a = a op b` with `a op= b`.

* Avoid unnecessary closures.

* Avoid .iter() and .iter_mut() for iterating over containers.

* Remove unneeded qualification.

lib/cretonne/src/ir/dfg.rs

@@ -247,10 +247,11 @@ impl DataFlowGraph {
         // Try to create short alias chains by finding the original source value.
         // This also avoids the creation of loops.
         let original = self.resolve_aliases(src);
-        assert!(dest != original,
-                "Aliasing {} to {} would create a loop",
-                dest,
-                src);
+        assert_ne!(dest,
+                   original,
+                   "Aliasing {} to {} would create a loop",
+                   dest,
+                   src);
         let ty = self.value_type(original);
         assert_eq!(self.value_type(dest),
                    ty,
@@ -326,8 +327,8 @@ pub enum ValueDef {
 impl ValueDef {
     /// Unwrap the instruction where the value was defined, or panic.
     pub fn unwrap_inst(&self) -> Inst {
-        match self {
-            &ValueDef::Res(inst, _) => inst,
+        match *self {
+            ValueDef::Res(inst, _) => inst,
             _ => panic!("Value is not an instruction result"),
         }
     }