Maps without keys should be given a 64-bit integer key of 0

Previously the auto-generated key type was the same as the value type,
which made maps containing string values use far more stack memory than
they should when accessing them.

src/codegen_llvm.cpp

@@ -400,7 +400,7 @@ AllocaInst *CodegenLLVM::getMapKey(Map &map)
   }
   else
   {
-    key = b_.CreateAllocaBPF(map.type, map.ident + "_key");
+    key = b_.CreateAllocaBPF(SizedType(Type::integer, 8), map.ident + "_key");
     b_.CreateStore(b_.getInt64(0), key);
   }
   return key;