separate DDict logic into its own module

created zstd_ddict.c within lib/decompress

lib/common/zstd_internal.h

@@ -41,6 +41,9 @@ extern "C" {
 
 /* ---- static assert (debug) --- */
 #define ZSTD_STATIC_ASSERT(c) DEBUG_STATIC_ASSERT(c)
+#define ZSTD_isError ERR_isError   /* for inlining */
+#define FSE_isError  ERR_isError
+#define HUF_isError  ERR_isError
 
 
 /*-*************************************