Attempt to split into multiple crates

[Berrysoft]

Closes #70

However, I don't think it's easy to do so. Many pub(crate) elements are turned into public.

TODO:

• Split the tests to multiple crates.
• Fix the doc tests.
• Unify the crate metadata.

@George-Miao Do you like this attempt?

[George-Miao]

BTW I think the branch should be named as refactor/split as it's not adding any new features.

[George-Miao]

Also, in compio-signal/src/lb.rs, ctrl_c is shown differently on windows and unix docs. On windows, it's no_inline-d so there's a separate pub use windows::ctrl_c while on unix it's defined in lib.rs. Maybe some consistency is desirable?

[George-Miao]

LGTM