Improve ergonomics for qualifier keys (#21)

* remove unnecessary constraint

* add impls for using QualifierKey as a string

* add without_qualifiers

purl/src/builder.rs

@@ -153,12 +153,17 @@ impl<T> GenericPurlBuilder<T> {
     pub fn without_qualifier<S>(mut self, k: S) -> Self
     where
         S: AsRef<str>,
-        SmallString: From<S>,
     {
         self.parts.qualifiers.remove(k);
         self
     }
 
+    /// Unset all qualifiers.
+    pub fn without_qualifiers(mut self) -> Self {
+        self.parts.qualifiers.clear();
+        self
+    }
+
     /// Set the subpath.
     ///
     /// Passing `""` will unset the subpath.

purl/src/qualifiers.rs

@@ -315,7 +315,7 @@ impl<'a> IntoIterator for &'a mut Qualifiers {
 /// A case-insensitive qualifier name.
 ///
 /// Comparisons between this type and other types are case insensitive.
-#[derive(Clone, Debug, Default, Eq, Hash, PartialEq, PartialOrd, Ord)]
+#[derive(Clone, Debug, Default, Eq, Hash, Ord)]
 pub struct QualifierKey(SmallString);
 
 impl<S> PartialEq<S> for QualifierKey
@@ -345,6 +345,24 @@ impl Deref for QualifierKey {
     }
 }
 
+impl AsRef<str> for QualifierKey {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl From<QualifierKey> for SmallString {
+    fn from(value: QualifierKey) -> Self {
+        SmallString::from(value.0)
+    }
+}
+
+impl From<&QualifierKey> for SmallString {
+    fn from(value: &QualifierKey) -> Self {
+        SmallString::from(value.as_str())
+    }
+}
+
 impl QualifierKey {
     /// Get a reference to the lower case string.
     pub fn as_str(&self) -> &str {