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/regalloc/pressure.rs

@@ -199,7 +199,7 @@ impl Pressure {
 
     /// Reset all counts to 0, both base and transient.
     pub fn reset(&mut self) {
-        for e in self.toprc.iter_mut() {
+        for e in &mut self.toprc {
             e.base_count = 0;
             e.transient_count = 0;
         }
@@ -220,7 +220,7 @@ impl Pressure {
 
     /// Reset all transient counts to 0.
     pub fn reset_transient(&mut self) {
-        for e in self.toprc.iter_mut() {
+        for e in &mut self.toprc {
             e.transient_count = 0;
         }
     }