v5.0 project plan

[sisong]

• define a new delta file format
• saved checksums of datas, & set default compressor(maybe zstd; can set -c-no to close it);
• greatly optimize memory require size for diff -m mode; (some speed and delta size as cost, ref #427 )
• fully compatible with VCDIFF format window constraints when diff -VCD;
• optimized patch speed for new delta file format; (add fixed memory require size as cost, the size set by diff)
• new patcher only used C, & support multi-thread; (ref #313 ) ；
• if the feature is suitable, it may be released in v4.x（or for existing delta format）;