Add Encode/Decode/BorrowDecode constraints for struct generic type parameters in derive macro's outputs

[songzhi]

For example:

// raw code
use bincode::Encode;

#[derive(Encode)]
struct Foo<T> {
    inner: T,
}

// current derive macro output, and it's failed to compile!
use bincode::Encode;
struct Foo<T> {
    inner: T,
}
impl<T> bincode::enc::Encode for Foo<T> {
    fn encode<E: bincode::enc::Encoder>(
        &self,
        mut encoder: E,
    ) -> core::result::Result<(), bincode::error::EncodeError> {
        bincode::enc::Encode::encode(&self.inner, &mut encoder)?;
        Ok(())
    }
}

// what it should ouput
use bincode::Encode;
struct Foo<T> {
    inner: T,
}
impl<T: Encode> bincode::enc::Encode for Foo<T> {
    fn encode<E: bincode::enc::Encoder>(
        &self,
        mut encoder: E,
    ) -> core::result::Result<(), bincode::error::EncodeError> {
        bincode::enc::Encode::encode(&self.inner, &mut encoder)?;
        Ok(())
    }
}

The current macro needs the type parameter T be T: Encode. But in Rust, the latter one is more common.
I think most of derive macros in Rust ecosystem like serde::Serialize do the latter way.
So it's better for users when bincode's derive macros do it so.

[VictorKoenders]

I agree with this change and have implemented it in #454.

In the future we might want to make a #[bincode(bounds = "...")] attribute that overrides this, like serde has.

Alternatively I think people can just implement their own implementation, it's a simple trait to implement.