Expand the unsafe & low-level guide to cover static mut, more #[no_std], modern asm!

[huonw]

The unsafe & low-level code guide added in #12887 should cover:

• transmute/casting
• the Unsafe type (for inner mutability)
• the relevance of the various kinds (i.e. Sync etc.), and when to apply the types in std::kinds::marker to ensure safety
• static mut, including atomics and static mutexes
• #[no_std] in more detail: I don't know enough about the correct compilation flags, and symbols required, so that section is rather loose on detail
• the asm! syntax described isn't 100% up-to-date; I took it from the description in the 0.6 release notes, and there have been extensions and improvements since that was written.

[flaper87]

Triage bump: still needs to be updated.

@steveklabnik Gentle heads up

[steveklabnik]

Okay, so at this point:

• transmute/casting - covered in the casting chapter
• the Unsafe type (for inner mutability) - would be nice to have in a guide, but api docs work too
• the relevance of the various kinds - same
• static mut, including atomics and static mutexes - covered in the statics chapter, could beef up atomics/mutexes though
• #[no_std] in more detail: - has its own chapter
• the asm! syntax - has its own chapter.

as such, I'm going to give this a close. Adding a "building unsafe abstractions safely" chapter would be nice for Effective Rust, but I'm not leaving tickets open for possible chapters right now.