mixin macros

This crate contains mixin macros that combines fields and implementations of different structs.

Example:

```rust

[mixin::declare]

pub struct Named { name: String, }

[mixin::expand]

impl Named { pub fn name(&self) -> &str { &self.name } }

[mixin::insert(Named)]

pub struct MyStruct {}

[test]

fn testit() { let mystruct = MyStruct { name: "MixIn Works" }; asserteq!(mystruct.name(), "MixIn Works"); } ```