Sort macro input tokens
Example
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After sorting, it is easy to input keys in an unordered manner
```rust
macro_rules! foo {
(@branch (a $a:literal) (b $b:literal)) => {
concat!(stringify!($a), stringify!($b))
};
($($name:ident : $val:literal)+) => {
sort_macro::sort_in!({@branch} foo!($( ($name $val) )+))
};
}
fn main() {
assert_eq!(foo!(b:3 a:2), "23");
assert_eq!(foo!(a:2 b:3), "23");
}
```
Grammar
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Sort in last group, sort key is `TokenTree::to_string`
```abnf
input = [prefix] *tt group
prefix = "{" *tt "}"
```