Adding documentation and explanations to some parts of core

[elichai]

Hi, I'm writing documentation for the core code,
right now I'm focusing things that answer 2/3 of these:

1. A non self explanatory complex code.
2. A code that I had written.
3. Areas in the code that right now I have most of the knowledge about them (i.e. weird rust/sgx juggling)

[elichai]

I removed this nice explanation after all that issue was resolved in #159

//! # Serde Implementations for KM primitives.
//!
//! The only reason why we need to implement Serialize/Deserialize ourselves is because the derive macro
//! contains `extern crate serde as _serde` but we renamed serde. so that's invalid. https://github.com/serde-rs/serde/pull/1499
//! We had to rename it to support 2 different serde implementations, 1 from crates.io and another from baidu.
//!
//! I tried to keep this implementation as simple as possible and as close as possible to the auto generated one <br>
//! (you can see the autogenerated one by running
//! `cargo rustc --profile=check -- -Zunstable-options --pretty=expanded` on a code with the derive macro)

[AvishaiW]

Very enriching! Thanks :)

[AvishaiW]

perfect!