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/extfunc.rs

@@ -70,7 +70,7 @@ impl Signature {
 pub struct DisplaySignature<'a>(&'a Signature, Option<&'a RegInfo>);
 
 fn write_list(f: &mut fmt::Formatter,
-              args: &Vec<ArgumentType>,
+              args: &[ArgumentType],
               regs: Option<&RegInfo>)
               -> fmt::Result {
     match args.split_first() {