Fix clippy lints surfaced by a newer toolchain

CI runs with an unpinned `@stable` Rust toolchain, and a newer clippy
than the last green `main` run flags pre-existing code:

  - collapsible_match in interpreter/interpret.rs and xslt-ast/
    whitespace.rs (an `if` inside a match arm folded into a guard)
  - useless_conversion in library/map.rs (redundant `.into_iter()`
    in two `Vec::extend` calls)

None of these are introduced by this branch -- they are unchanged from
`main` -- but they fail `cargo clippy -D warnings`, so fix them here to
unblock CI. The durable fix is upstream pinning the toolchain.

Author: Wietse

xee-interpreter/src/interpreter/interpret.rs

@@ -1130,13 +1130,11 @@ impl<'a> Interpreter<'a> {
                                 "Appending a whole document is not supported yet",
                             )));
                         }
-                        xot::Value::Text(text) => {
+                        xot::Value::Text(text) if text.get().is_empty() => {
                             // zero length text nodes are skipped
                             // Can this even exist, or does Xot not have
                             // them anyway?
-                            if text.get().is_empty() {
-                                continue;
-                            }
+                            continue;
                         }
                         _ => {}
                     }

xee-interpreter/src/library/map.rs

@@ -162,7 +162,7 @@ fn find_helper(
                 function::Function::Array(array) => {
                     for entry in array.iter() {
                         let found = find_helper(entry, key.clone())?;
-                        result.extend(found.into_iter())
+                        result.extend(found)
                     }
                 }
                 function::Function::Map(map) => {
@@ -171,7 +171,7 @@ fn find_helper(
                             result.push(v.clone());
                         }
                         let found = find_helper(v, key.clone())?;
-                        result.extend(found.into_iter())
+                        result.extend(found)
                     }
                 }
                 _ => {}

xee-xslt-ast/src/whitespace.rs

@@ -14,12 +14,11 @@ pub(crate) fn strip_whitespace(xot: &mut Xot, names: &Names, node: Node) {
     for edge in xot.traverse(node) {
         match edge {
             NodeEdge::Start(node) => match xot.value(node) {
-                Value::Text(text) => {
+                Value::Text(text)
                     if is_xml_whitespace(text.get())
-                        && !is_xml_space_preserve(xot, names, node, &xml_space_preserve)
-                    {
-                        to_remove.push(node);
-                    }
+                        && !is_xml_space_preserve(xot, names, node, &xml_space_preserve) =>
+                {
+                    to_remove.push(node);
                 }
                 Value::Element(_) => {
                     if let Some(xml_space) = xot.attributes(node).get(xot.xml_space_name()) {