Encode common safety anti-patterns in Clippy

[Shnatsel]

https://github.com/rust-lang/rust-clippy is the go-to linter for Rust. We should encode common anti-patterns in it so that people will get warnings about such code. This includes:

1. Code that compromises safety directly, such as improper path handling
2. Code that tends to be slow and get refactored into unsafe code for performance reasons, even though a safe alternative is available (see New lint: slow or unsafe vector initialization rust-lang/rust-clippy#3237 for an example)

#19 will help in detecting the anti-patterns.

[tarcieri]

Whoops, guess I opened #27 without seeing this before and it's a dup. How would you like to proceed @Shnatsel ?

[Shnatsel]

This is now a duplicate of #27, closing.