Closes #90.

This PR implements lint groups! These, just like Clippy's groups, allow enabling and disable multiple lints at a time based on their categories. I, stealing Clippy's groups, have added:

• Correctness: Zero-false-positives deny-by-default "if you hit these, you're wrong" lints
• Suspicious: Like Correctness, but the behavior causing the lint may actually be intended. These are warnings, not hard errors.
• Complexity: Hey, you! That while you wrote there could be a for loop! (This category checks for overly-complex code with good alternatives.)
• Performance: Anything that can be changed to improve performance. (No, this doesn't warn if you call std::thread::sleep(), that's your own fault.)
• Style: By far the most controversial of them all, this category enforces "idiomatic Bevy code". What does that mean? Fantastic question, let's bikeshed this into oblivion!
  • In all serious, because this is on-by-default, I'm going to be very conservative adding anything to this category. Serious thought is required!
• Pedantic: Similar to Style, but disabled by default. This category is the nit-picky math teacher you had that one time who hated shortcuts, calculators, and mental math.
• Restriction: Opt-in lints that you may want for specific purposes, such as requiring all components to derive Reflect.
• Nursery: A category for unstable and flaky lints that may be knighted one day into full-on lints. For now, this category should not be used, since bevy_lint hasn't been released yet!

As part of this PR, I've changed a few things about how lints are defined.

1. Lints are declared with the declare_bevy_lint! macro. This is very similar to the declare_tool_lint! that we've used before.
2. BevyLint struct: contains the Lint struct and a reference to the lint group this is a member of.
3. LintGroup struct: represents the name and default level (deny, warn, allow) of a lint.

#![feature(register_tool)]
#![register_tool(bevy)]

#![deny(bevy::style)]

use bevy::prelude::*;

fn main() {
    App::new().run();
}

warning: unknown lint: `bevy::style`
 --> bevy_lint/examples/lint_test.rs:4:9
  |
4 | #![deny(bevy::style)]
  |         ^^^^^^^^^^^
  |
  = note: `#[warn(unknown_lints)]` on by default

warning: an entrypoint that calls `App::run()` does not return `AppExit`
 --> bevy_lint/examples/lint_test.rs:9:16
  |
8 | fn main() {
  |          - help: try: `-> AppExit`
9 |     App::new().run();
  |                ^^^^^
  |
  = note: `App::run()` returns `AppExit`, which can be used to determine whether the app exited successfully or not
  = note: `#[warn(bevy::main_return_without_appexit)]` on by default

warning: `bevy_lint` (example "lint_test") generated 2 warnings