See #201

• Support for macOS with Apple Silicon (Darwin/ARM64)
• Requires Go 1.16 to build

Experimental generic TCP corking (user-space buffering) to coalesce the first few packets sent by both go-ss2 client and server. It would make the packet length pattern less obvious.

1. Fixed the toggle to disable TCP mode.
2. Experimental TCP Cork to coalesce salt/address/initial payload.

Added -udp and -tcp to toggle UDP/TCP mode on server-side. By default TCP mode is enabled and UDP mode is disabled.

Dropped all unsafe stream ciphers, and added Bloom filters to defend against replay attack.