Move backtracking for implements to specific spaces case that it's actually needed for, to avoid excess parsing work in extreme cases

crates/compiler/parse/src/blankspace.rs

@@ -186,7 +186,7 @@ where
     E: 'a + SpaceProblem,
 {
     parser::map(
-        and(space0_e(indent_problem), parser),
+        and(backtrackable(space0_e(indent_problem)), parser),
         |(space_list, item): (&'a [CommentOrNewline<'a>], S)| SpacesBefore {
             before: space_list,
             item,

crates/compiler/parse/src/expr.rs

@@ -1163,10 +1163,7 @@ fn opaque_signature<'a>(
     and(
         specialize_err(EExpr::Type, type_annotation::located_opaque_signature(true)),
         optional(map_with_arena(
-            backtrackable(specialize_err(
-                EExpr::Type,
-                type_annotation::implements_abilities(),
-            )),
+            specialize_err(EExpr::Type, type_annotation::implements_abilities()),
             |arena, item| &*arena.alloc(item),
         )),
     )