Support widestring macro on non-literal strings

This enables usage in macros like

        L!(stringify!($snake_case_name))

in the upcoming AST port.

fish-rust/src/wchar.rs

@@ -14,7 +14,7 @@
 /// The result is of type wstr.
 /// It is NOT nul-terminated.
 macro_rules! L {
-    ($string:literal) => {
+    ($string:expr) => {
         widestring::utf32str!($string)
     };
 }