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/Lz4Encoder.h

@@ -29,7 +29,6 @@ struct CProps
   Byte _ver_major;
   Byte _ver_minor;
   Byte _level;
-  Byte _reserved[2];
 };
 
 class CEncoder: