Clean up the codebase

[tatetian]

Here is a list of suggestions to clean up the codebase.

• Project-level changes
  • Third-party dependencies
    • Fork the repo of third-party crates so that we can keep track of the upstream changes (see occlum/io-uring)
    • Move all third-party crates into a separate dir
    • Manage the dependencies on third-party crates with Git submodule (except rust-sgx-sdk, which is too big)
  • Naming conventions:
    • Use - for crate names; otherwise, use _.
    • Prefix a crate's name with sgx- if the crate can only work on SGX
  • Move benchmarks and examples to the corresponding folders in each sub-project
• Code-level changes
  • Use feature gates to switch between SGX or non-SGX version (see occlum/io-uring)
  • Simplify the use of cfg
  • Add README to sub-projects
    • Including instructions about building and running samples or benches

After the clean-up, the structure of the codebase will look like below:

async-rt/
    ...
async-file/
    src/
    examples/
        sgx/
    benches/
        sgx/
    README.md
io-uring-callack/
    src/
    README.md
sgx-untrusted-alloc/
    README.md
thirt_parties/
    slab/
    flume/
    rust-sgx-sdk/
    io-uring/
        src/
        ocalls/

[tatetian]

Done