Fix the parser, make the function name optional.

src/syntax/parser.lalrpop

@@ -76,7 +76,14 @@ pub TopLevel: TopLevel = {
 }
 
 Function: TopLevel = {
-    "function" "(" Arguments OptionalComma ")" Expression => unimplemented!(),
+    "function" <opt_name:OptionalName> "(" <args:Arguments> OptionalComma ")" <exp:Expression> =>
+        TopLevel::Function(opt_name, args, exp),
+}
+
+OptionalName: Option<Name> = {
+    <name_start: @L> <v:"<var>"> <name_end: @L> =>
+        Some(Name::new(v, Location::new(file_idx, name_start..name_end))),
+    => None,
 }
 
 Arguments: Vec<Name> = {