Macros: Add a 'literal' fragment specifier
See: https://github.com/rust-lang/rust/issues/35625
```rust
macro_rules! test_literal {
($l:literal) => {
println!("literal: {}", $l);
};
($e:expr) => {
println!("expr: {}", $e);
};
}
fn main() {
let a = 1;
test_literal!(a);
test_literal!(2);
test_literal!(-3);
}
```
Output:
```
expr: 1
literal: 2
literal: -3
```
ToDo:
* [x] Feature gate
* [x] Basic tests
* [x] Tests for range patterns
* [x] Tests for attributes
* [x] Documentation
* [x] Fix for `true`/`false`
* [x] Fix for negative number literals