refactor: integrate arceos_posix_api

[AsakuraMizu]

Current Status

• To be discussed

Description

Following #22, this PR is a implementation of the proposal #16. This PR gets rid of the dependency on arceos_posix_api of Starry.

Goals

• Safe: write safe code instead of passing pointers and using unsafes as much as possible.
• Developer friendly: we no longer need to do dirty works in two repositories simultaneously.
• Ultimately, we can merge our progress into upstream repo.

Not yet ported from arceos_posix_api

• Pthread (do we need it?)
• I/O multiplexing
• Networking syscalls
• Misc syscalls, e.g. sysconf and getrlimit

I recommend doing them in separate PRs so as not to make this huge PR even bigger.

[AsakuraMizu]

Previously at #22 (comment), we decided to put the file descriptor table in api. However, the biggest obstacle is that when cloning tasks, it needs to be cloned or shared, and this part is done in core.

A possible solution: split part of the code for cloning and spawning tasks into api, including flags handling, namespace initializing etc.