safe-abstraction: Apply to granule-locked structs

[bitboom]

This PR applies safe abstraction to structures managed with granule locks.

1. The affected structures are RD, RTT and REC.
2. content_mut has been modified to manage &mut in accordance with Rust's syntax.
3. Four unsafe keyword have been removed.

[bokdeuk-jeong]

@bitboom , @jinbpark It's a long overdue question, how can we relate it to the intern's task?

[jinbpark]

What the intern is working on is a dead-lock detection. It would look like a spinlock library (which can replace spinning_top::Spinlock) having the ability to detect a dead-lock situation.