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/Lz4Decoder.cpp

@@ -87,12 +87,16 @@ STDMETHODIMP CDecoder::SetDecoderProperties2(const Byte * prop, UInt32 size)
 {
   DProps *pProps = (DProps *)prop;
 
-  if (size != sizeof(DProps))
+  switch (size) {
+  case 3:
+    memcpy(&_props, pProps, 3);
+    return S_OK;
+  case 5:
+    memcpy(&_props, pProps, 5);
+    return S_OK;
+  default:
     return E_NOTIMPL;
-
-  memcpy(&_props, pProps, sizeof (DProps));
-
-  return S_OK;
+  }
 }
 
 STDMETHODIMP CDecoder::SetNumberOfThreads(UInt32 numThreads)