`Chain::read`/`write` should require `T: FromBytes`/`IntoBytes`

[FelixMcFelix]

The current trait bounds here are too lax, allowing the compiler to accept code such as:

fn ctl_test(
    chain: &mut Chain,
    mem: &MemCtx,
) -> Result<(), ()> {
    let mut a = false;
    if !chain.read(&mut a, &mem) {
        return Err(());
    }
    Ok(())
}

I think every use of this API is using well-formed types (so isn't liable to hit UB on an invalid value), but we can rely on the typesystem to keep us right.

The underlying call to MemCtx::read already requires FromBytes, while MemCtx::write requires only Copy. It should probably also require IntoBytes for symmetry.

[iximeow]

(done in #1130)