Update FStringElements to deref to a slice (#11570)

Ref: #11400 (comment)

Author: dhruvmanila

crates/ruff_python_ast/src/nodes.rs

@@ -1580,7 +1580,7 @@ impl<'a> IntoIterator for &'a mut FStringElements {
 }
 
 impl Deref for FStringElements {
-    type Target = Vec<FStringElement>;
+    type Target = [FStringElement];
 
     fn deref(&self) -> &Self::Target {
         &self.0

crates/ruff_python_parser/src/parser/expression.rs

@@ -1298,7 +1298,7 @@ impl<'src> Parser<'src> {
     ///
     /// If the parser isn't positioned at a `{` or `FStringMiddle` token.
     fn parse_fstring_elements(&mut self) -> FStringElements {
-        let mut elements = FStringElements::default();
+        let mut elements = vec![];
 
         self.parse_list(RecoveryContextKind::FStringElements, |parser| {
             let element = match parser.current_token_kind() {
@@ -1348,7 +1348,7 @@ impl<'src> Parser<'src> {
             elements.push(element);
         });
 
-        elements
+        FStringElements::from(elements)
     }
 
     /// Parses a f-string expression element.