expression: set integer literals as int by default

vlib/builtin/int_test.v

@@ -150,8 +150,9 @@ fn test_int_decl() {
 	assert typeof(x3) == 'int'
 	assert typeof(x4) == 'int'
 	assert typeof(x5) == 'int'
+	// integers are always 'int' by default
 	x6 := 989898932113111
-	x7 := -321314588900011
-	assert typeof(x6) == 'u64'
+	x7 := u64(-321314588900011)
+	assert typeof(x6) == 'int'
 	assert typeof(x7) == 'u64'
 }