changed initStatic?Dict() return type to const ZSTD_?Dict*

ZSTD_create?Dict() is required to produce a ?Dict* return type
because `free()` does not accept a `const type*` argument.
If it wasn't for this restriction, I would have preferred to create a `const ?Dict*` object
to emphasize the fact that, once created, a dictionary never changes
(hence can be shared concurrently until the end of its lifetime).

There is no such limitation with initStatic?Dict() :
as stated in the doc, there is no corresponding free() function,
since `workspace` is provided, hence allocated, externally,
it can only be free() externally.

Which means, ZSTD_initStatic?Dict() can return a `const ZSTD_?Dict*` pointer.

Tested with `make all`, to catch initStatic's users,
which, incidentally, also updated zstd.h documentation.

lib/compress/zstd_compress.c

@@ -2634,7 +2634,8 @@ size_t ZSTD_freeCDict(ZSTD_CDict* cdict)
  *  Note : there is no corresponding "free" function.
  *         Since workspace was allocated externally, it must be freed externally.
  */
-ZSTD_CDict* ZSTD_initStaticCDict(void* workspace, size_t workspaceSize,
+const ZSTD_CDict* ZSTD_initStaticCDict(
+                                 void* workspace, size_t workspaceSize,
                            const void* dict, size_t dictSize,
                                  ZSTD_dictLoadMethod_e dictLoadMethod,
                                  ZSTD_dictMode_e dictMode,

lib/decompress/zstd_decompress.c

@@ -2139,9 +2139,10 @@ ZSTD_DDict* ZSTD_createDDict_byReference(const void* dictBuffer, size_t dictSize
 }
 
 
-ZSTD_DDict* ZSTD_initStaticDDict(void* workspace, size_t workspaceSize,
-                                 const void* dict, size_t dictSize,
-                                 ZSTD_dictLoadMethod_e dictLoadMethod)
+const ZSTD_DDict* ZSTD_initStaticDDict(
+                                void* workspace, size_t workspaceSize,
+                                const void* dict, size_t dictSize,
+                                ZSTD_dictLoadMethod_e dictLoadMethod)
 {
     size_t const neededSpace =
             sizeof(ZSTD_DDict) + (dictLoadMethod == ZSTD_dlm_byRef ? 0 : dictSize);

lib/zstd.h

@@ -568,15 +568,17 @@ ZSTDLIB_API ZSTD_CStream* ZSTD_initStaticCStream(void* workspace, size_t workspa
 ZSTDLIB_API ZSTD_DCtx*    ZSTD_initStaticDCtx(void* workspace, size_t workspaceSize);
 ZSTDLIB_API ZSTD_DStream* ZSTD_initStaticDStream(void* workspace, size_t workspaceSize);    /**< same as ZSTD_initStaticDCtx() */
 
-ZSTDLIB_API ZSTD_CDict* ZSTD_initStaticCDict(void* workspace, size_t workspaceSize,
-                                             const void* dict, size_t dictSize,
-                                             ZSTD_dictLoadMethod_e dictLoadMethod,
-                                             ZSTD_dictMode_e dictMode,
-                                             ZSTD_compressionParameters cParams);
+ZSTDLIB_API const ZSTD_CDict* ZSTD_initStaticCDict(
+                                        void* workspace, size_t workspaceSize,
+                                        const void* dict, size_t dictSize,
+                                        ZSTD_dictLoadMethod_e dictLoadMethod,
+                                        ZSTD_dictMode_e dictMode,
+                                        ZSTD_compressionParameters cParams);
 
-ZSTDLIB_API ZSTD_DDict* ZSTD_initStaticDDict(void* workspace, size_t workspaceSize,
-                                             const void* dict, size_t dictSize,
-                                             ZSTD_dictLoadMethod_e dictLoadMethod);
+ZSTDLIB_API const ZSTD_DDict* ZSTD_initStaticDDict(
+                                        void* workspace, size_t workspaceSize,
+                                        const void* dict, size_t dictSize,
+                                        ZSTD_dictLoadMethod_e dictLoadMethod);
 
 /*! Custom memory allocation :
  *  These prototypes make it possible to pass your own allocation/free functions.