Reduce again the number of match calls (#7815)

- Reduce the number of match calls (see commit messages)
- A few miscellaneous improvements

crates/nu-command/src/dataframe/eager/list.rs

@@ -51,9 +51,8 @@ impl Command for ListDF {
 
         let vals = vals
             .into_iter()
-            .filter_map(|(name, value)| match NuDataFrame::try_from_value(value) {
-                Ok(df) => Some((name, df)),
-                Err(_) => None,
+            .filter_map(|(name, value)| {
+                NuDataFrame::try_from_value(value).ok().map(|df| (name, df))
             })
             .map(|(name, df)| {
                 let name = Value::String {

crates/nu-command/src/dataframe/eager/open.rs

@@ -103,14 +103,13 @@ fn command(
 
     let type_id = match &type_option {
         Some(ref t) => Some((t.item.to_owned(), "Invalid type", t.span)),
-        None => match file.item.extension() {
-            Some(e) => Some((
+        None => file.item.extension().map(|e| {
+            (
                 e.to_string_lossy().into_owned(),
                 "Invalid extension",
                 file.span,
-            )),
-            None => None,
-        },
+            )
+        }),
     };
 
     match type_id {