[lib] Allow ZSTD_CCtx_loadDictionary() to be called before parameters are set

* After loading a dictionary only create the cdict once we've started the
  compression job. This allows the user to pass the dictionary before they
  set other settings, and is in line with the rest of the API.
* Add tests that mix the 3 dictionary loading APIs.
* Add extra tests for `ZSTD_CCtx_loadDictionary()`.
* The first 2 tests added fail before this patch.
* Run the regression test suite.

lib/compress/zstd_compress_internal.h

@@ -54,6 +54,14 @@ typedef struct ZSTD_prefixDict_s {
     ZSTD_dictContentType_e dictContentType;
 } ZSTD_prefixDict;
 
+typedef struct {
+    void* dictBuffer;
+    void const* dict;
+    size_t dictSize;
+    ZSTD_dictContentType_e dictContentType;
+    ZSTD_CDict* cdict;
+} ZSTD_localDict;
+
 typedef struct {
     U32 CTable[HUF_CTABLE_SIZE_U32(255)];
     HUF_repeat repeatMode;
@@ -245,7 +253,7 @@ struct ZSTD_CCtx_s {
     U32    frameEnded;
 
     /* Dictionary */
-    ZSTD_CDict* cdictLocal;
+    ZSTD_localDict localDict;
     const ZSTD_CDict* cdict;
     ZSTD_prefixDict prefixDict;   /* single-usage dictionary */