Extend TryFromBytes to support validation context

[joshlf]

See also: #5 (comment)

This is an extension of the design described in #5

TODO: Fill out details

pub unsafe trait TryFromBytes {
    // This can either default to `()` by dint of the custom derive emitting that,
    // or we can use associated type defaults once they're stable.
    #[doc(hidden)]
    type Context: Default;

    #[doc(hidden)]
    fn is_bit_valid(ctx: &mut Self::Context, ptr: NonNull<Self>) -> bool;

    fn try_read_from(bytes: &[u8]) -> Option<Self> where Self: Sized {
        let maybe_uninit = MaybeUninit::<Self>::read_from(bytes)?;
        let ptr = NonNull::from(&maybe_uninit).cast::<Self>();
        let mut ctx = <Self::Context as Default>::default();
        if Self::is_bit_valid(&mut ctx, ptr) {
            unsafe { maybe_uninit.assume_init() }
        }
    }
}

Per @djkoloski, for rkyv's use case, the Context type might actually need to be constructible from the byte slice that's being parsed from, ie type Context: for<'a> From<&'a [u8]>.