Refactor process and channel code

[yihuaf]

1. Refactor the main process related code into its own mod.
2. Implemented channel with socketpair. The channel can now send anything that implements serialize and deserialize, making it much more flexible. The channel can also send file descriptors across process now. This will be needed when implementing seccomp notify. Without this, we will have to use pidfd_getfd syscall, which only available in kernel > 5.6.

Submitting this PR before seccomp notify change, since this will make the review much easier.

[codecov-commenter]

Codecov Report

Merging #388 (e1ae89f) into main (5ae2385) will increase coverage by 0.73%.
The diff coverage is 72.97%.

@@            Coverage Diff             @@
##             main     #388      +/-   ##
==========================================
+ Coverage   76.34%   77.07%   +0.73%     
==========================================
  Files          52       52              
  Lines        8306     8336      +30     
==========================================
+ Hits         6341     6425      +84     
+ Misses       1965     1911      -54     

[utam0k]

Awesome