Implement `Segment`, `Imap`

[travis1829]

• Added segment.rs and imap.rs (Probably almost everything are at least functional). To see how they works, see inode.rs as an example.
• inode.rs and mod.rs still needs a lot of changes, but doesn't seem that hard.

[travis1829]

After implementing the Lfs, we should refactor it using some new types.

• Suggestion: SegBlockGuard
  • We should only read from a Buf, and only write to a SegBlockGuard.
  • Can obtain a SegBlockGuard only from the Segment's methods.
  • When a SegBlockGuard drops, automatically commits the Segment if it was full.
  • This is because we need to commit the Segment after the writing is done.
  • For Buf, maybe we want to add a new type, or make Buf::deref_inner_mut usable only when #[cfg(feature = "ufs")].

[jeehoonkang]

bors r+

[kaist-cp-bors[bot]]

Build succeeded:

• ci