lang: Prevent struct types with duplicate field names

The previous fix for #591 in 70eecd5 didn't address all issues
concerning duplicate struct field names. It still crashed for inputs
like `$d []struct{x int; x float}`. Note the different types but
duplicate names.

lang/interpret_test/TestAstFunc2/struct-duplicate4/main.mcl

@@ -0,0 +1 @@
+$d []struct{x int;x float}