Attribute: "read_fn" / "write_fn"

[neuronsguy]

Hi,

I'm trying to use speedy which is a great fit for my use-case, but running into the issue of being unable to implement Readable/Writable structs in crates that I don't own.

It would be very handy to have an attribute like #[speedy(read_fn = my_reader_fn)] that would allow defining a custom function to turn a given field into bytes.

Rough example API:

#[derive(Readable,Writable)]
struct Mine {
    field1: u64,
    #[speedy(read_fn=read_other_crate_struct)]
    #[speedy(write_fn=write_other_crate_struct)]
    field2: OtherCrateStruct
}

fn write_other_crate_struct<T: ?Sized + Writer<C>,C:Context>(val: &OtherCrateStruct, writer: &mut T) -> Result<(), C::Error> {
    todo!()
}

fn read_other_crate_struct<R: Reader<C>,C:Context>(reader: &mut R) -> Result<OtherCrateStruct, C::Error> {
    todo!()
}

This would remove the need to fork crates to add Readable/Writable implementations to them.

If this change is easy to make I'd be happy to do it, although I'm not familiar with macro / derive code so would need some pointers.