Partial safety comments

[Manishearth]

Progress on #98

Figured I'd land what I have so far.

I still need to do:

• mem.rs (partial)
• lib.rs
• utf_8.rs

Furthermore, I've not done too much commenting in handles.rs: I think there are some refactors to that code that may reduce the number of tricky invariants.

There's also a safety todo around endianness in ascii.rs.

The general methodology I'm using is:

• document every unsafe block
• if the unsafe block relies on invariants that are not immediately checked above it, document that invariant on the relevant variable (and if it is mutated, on every mutation point)
• make sure all functions have safety invariants documented

[Manishearth]

r? @hsivonen

[hsivonen]

Thank you!