Rewrite attribute parser using parse_nested_meta

test_suite/tests/ui/malformed/trailing_expr.stderr

@@ -1,5 +1,5 @@
-error: expected `,`
- --> tests/ui/malformed/trailing_expr.rs:5:35
+error: expected `=`
+ --> tests/ui/malformed/trailing_expr.rs:5:32
   |
 5 |     #[serde(skip_serializing_if, x.is_empty())]
-  |                                   ^
+  |                                ^

test_suite/tests/ui/unexpected-literal/container.stderr

@@ -1,4 +1,4 @@
-error: expected path
+error: unexpected literal in nested attribute, expected ident
  --> tests/ui/unexpected-literal/container.rs:4:9
   |
 4 | #[serde("literal")]

test_suite/tests/ui/unexpected-literal/field.stderr

@@ -1,4 +1,4 @@
-error: expected path
+error: unexpected literal in nested attribute, expected ident
  --> tests/ui/unexpected-literal/field.rs:5:13
   |
 5 |     #[serde("literal")]

test_suite/tests/ui/unexpected-literal/variant.stderr

@@ -1,4 +1,4 @@
-error: expected path
+error: unexpected literal in nested attribute, expected ident
  --> tests/ui/unexpected-literal/variant.rs:5:13
   |
 5 |     #[serde("literal")]