Add zstd fast levels and update 7z property sizes

- add the "fast compression levels" of zstd via "fast" option (fast=1..64)
- change the 7-Zip property sizes of LZ4, LZ5 and Zstandard to 3
- 3 and 5 byte header are valid now (default is 3)
- update the Methods-Extern.md file, to reflect the property changes

CPP/7zip/Compress/ZstdDecoder.cpp

@@ -39,8 +39,10 @@ STDMETHODIMP CDecoder::SetDecoderProperties2(const Byte * prop, UInt32 size)
 
   switch (size) {
   case 3:
+    memcpy(&_props, pProps, 3);
+    return S_OK;
   case 5:
-    memcpy(&_props, pProps, sizeof (DProps));
+    memcpy(&_props, pProps, 5);
     return S_OK;
   default:
     return E_NOTIMPL;