introduce func compactor.

$(eval ## comment) will be nop when parsed.
diff --git a/expr_test.go b/expr_test.go
index 83b9b26..e21d66c 100644
--- a/expr_test.go
+++ b/expr_test.go
@@ -204,6 +204,12 @@
 				},
 			},
 		},
+		{
+			in: `$(eval ## comment)`,
+			val: &funcNop{
+				expr: `$(eval ## comment)`,
+			},
+		},
 	} {
 		val, _, err := parseExpr([]byte(tc.in), nil)
 		if tc.isErr {