Fix clippy reported problems of len_zero lint

This change fixes occurrences flagged by the clippy lint `len_zero`.
E.g.,

> warning: length comparison to one
>    --> src/parser.rs:694:12
>     |
> 694 |         if names.len() < 1 {
>     |            ^^^^^^^^^^^^^^^ help: using `is_empty` is clearer and more explicit: `names.is_empty()`
>     |
>     = help: for further information visit
>       https://rust-lang-nursery.github.io/rust-clippy/master/index.html#len_zero

Author: d-e-s-o

src/parser.rs

@@ -515,7 +515,7 @@ impl<'parser, 'refer, T> Ref<'parser, 'refer, T> {
     {
         {
             let var = &mut self.parser.vars[self.varid];
-            if var.metavar.len() == 0 {
+            if var.metavar.is_empty() {
                 let mut longest_name = names[0];
                 let mut llen = longest_name.len();
                 for name in names.iter() {
@@ -565,7 +565,7 @@ impl<'parser, 'refer, T> Ref<'parser, 'refer, T> {
         }
         {
             let var = &mut self.parser.vars[self.varid];
-            if var.metavar.len() == 0 {
+            if var.metavar.is_empty() {
                 var.metavar = name.to_string();
             }
         }
@@ -691,7 +691,7 @@ impl<'parser> ArgumentParser<'parser> {
             action: action,
             });
 
-        if names.len() < 1 {
+        if names.is_empty() {
             panic!("At least one name for option must be specified");
         }
         for nameptr in names.iter() {
@@ -740,7 +740,7 @@ impl<'parser> ArgumentParser<'parser> {
         stdout: &mut Write, stderr: &mut Write)
         -> Result<(), i32>
     {
-        let name = if args.len() > 0 { &args[0][..] } else { "unknown" };
+        let name = if !args.is_empty() { &args[0][..] } else { "unknown" };
         match Context::parse(self, &args, stderr) {
             Parsed => return Ok(()),
             Exit => return Err(0),
@@ -892,11 +892,11 @@ impl<'a, 'b> HelpFormatter<'a, 'b> {
     fn write_help(&mut self) -> IoResult<()> {
         try!(self.write_usage());
         try!(write!(self.buf, "\n"));
-        if self.parser.description.len() > 0 {
+        if !self.parser.description.is_empty() {
             try!(wrap_text(self.buf, self.parser.description,TOTAL_WIDTH, 0));
             try!(write!(self.buf, "\n"));
         }
-        if self.parser.arguments.len() > 0
+        if !self.parser.arguments.is_empty()
             || self.parser.catchall_argument.is_some()
         {
             try!(write!(self.buf, "\nPositional arguments:\n"));
@@ -910,8 +910,8 @@ impl<'a, 'b> HelpFormatter<'a, 'b> {
                 None => {}
             }
         }
-        if self.parser.short_options.len() > 0
-            || self.parser.long_options.len() > 0
+        if !self.parser.short_options.is_empty()
+            || !self.parser.long_options.is_empty()
         {
             try!(write!(self.buf, "\nOptional arguments:\n"));
             for opt in self.parser.options.iter() {
@@ -924,7 +924,7 @@ impl<'a, 'b> HelpFormatter<'a, 'b> {
     fn write_usage(&mut self) -> IoResult<()> {
         try!(write!(self.buf, "Usage:\n  "));
         try!(write!(self.buf, "{}", self.name));
-        if self.parser.options.len() != 0 {
+        if !self.parser.options.is_empty() {
             if self.parser.short_options.len() > 1
                 || self.parser.long_options.len() > 1
             {