Fix clippy warnings

Add VirtualAddressRange::is_empty function and PagePoolArea default
implementation.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ibf029ea23e7c6759af36bdca90e704a95ea8a963
diff --git a/src/address.rs b/src/address.rs
index 3a35c6c..b608365 100644
--- a/src/address.rs
+++ b/src/address.rs
@@ -189,11 +189,16 @@
         )
     }
 
-    /// The length of the range in bytes
+    /// The length of the range in bytes.
     pub fn len(&self) -> Option<usize> {
         self.end.diff(self.start)
     }
 
+    /// Return true if it is an empty range.
+    pub fn is_empty(&self) -> bool {
+        self.start == self.end
+    }
+
     /// Create an iterator which returns virtual addresses in the range of the given step in bytes.
     pub fn step_by(self, step: usize) -> VirtualAddressIterator {
         VirtualAddressIterator {
diff --git a/src/page_pool.rs b/src/page_pool.rs
index d767caf..fbbcfb5 100644
--- a/src/page_pool.rs
+++ b/src/page_pool.rs
@@ -34,6 +34,12 @@
     }
 }
 
+impl<const AREA_SIZE: usize> Default for PagePoolArea<AREA_SIZE> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 /// Continuous pages
 pub struct Pages {
     pa: usize,