Replace i32 with u32

[dmitmel]

Support for negative coordinates has been removed in commit a0e23a3, but a lot of functions and constants still use signed integers for coordinates (e.g.: why do you need signed integers in layout constants?). So, I've replaced all i32s with u32s where it's possible.

This PR consists of one large commit because if you change an argument type in one function, everything breaks. So, here are notes about changes in each file (skip means that a file contains insignificant changes that can be skipped during review):

• game/console.rs – changed only one cast (skip)
• game/game_state/state_explore.rs – changed only one function and added player position underflow protection
• game/layout.rs – replaced all i32s with u32s (skip)
• game/mod.rs – changed only two lines in Game::draw_logo (skip)
• game/player.rs – changed the type of player's position (skip)
• game/world/chunk.rs – refactored render function so that it can work with unsigned integers (this change has been tested, feel free to ask any questions about it)
• game/world/mod.rs – replaced types and added chunk position underflow protection
• graphics/renderer.rs – replaced all i32s with u32s, improved border rendering
• graphics/sprite.rs – replaced all i32s with u32s (skip)
• maths/vector.rs:
  1. changed types in constants
  2. removed cast function
  3. implemented Neg trait
  4. added map function
  5. added function for conversion from Vector2D<u32> to Vector2D<i32>
  6. added add_signed function for adding Vector2D<i32> to Vector2D<u32>

[dmitmel]

@Hopson97 I've opened this PR a week ago, but there's no changes/discussion about it. So here are some questions:

1. Have you seen this PR?
2. If so, do you think it is relevant now? (I can prove this if you want, but I'm not being forceful, feel free to disagree)
3. If it is relevant, should I rebase it onto master?

[Hopson97]

I understand the motivation for making i32 into uxx, but I don't see the advantage of doing this.

While i32 can be negative, uxx is able to underflow, so that is still an issue.

u32 allows for a greater range of numbers, but I doubt I'll be using over 2 billion coordinates. 😄

[dmitmel]

@Hopson97 Ok, but this PR contains some refactors for chunk rendering logic to work with unsigned types. I'll open a new PR just for that.