prepare 1.2.1

Move changelog out of README.md.Merge boost.context 1.71.0

boost.context: https://www.boost.org/users/history/version_1_71_0.html
  Add support for RISC-V LP64D
  [#72](boostorg/context#72): Fix ABI violation on ppc64 ELFv2
  [#99](boostorg/context#99): cleanup of stack if creating ucontext fails

fix path for libcopp-config.cmake

fix and use lib64 for build script samples

break after alloc failed in benchmark

反向复用stack，尽量减少cache miss

merge boost 1.63.0

1. cmake标记为3.3以上

2. 修复嵌套jump协程时状态异常问题

fix vc compile