Auto-forward Order-typed ctx parameter as bit_order

[sachaw]

Hi, I hit the following issue in one of my projects.
When a nested struct/enum receives bit ordering via ctx = "o: Order", it currently has to also set bit_order = "o" for that order to be applied — otherwise the ctx parameter is silently ignored and codegen falls back to msb.
#602 hit this and was closed pointing to that duplicate-attribute workaround; this PR forwards an Order-typed ctx parameter as the runtime bit_order automatically when no explicit bit_order is given.

Repro (against 0.20.3)

use deku::prelude::*;

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[deku(bit_order = "lsb")]
struct Parent { left: Child, right: Child }

#[derive(Debug, PartialEq, DekuRead, DekuWrite)]
#[repr(u8)]
#[deku(bits = 4, id_type = "u8",
       ctx = "_o: deku::ctx::Order",
       ctx_default = "deku::ctx::Order::Lsb0")]
enum Child { Empty = 0x0, Full = 0xf }

#[test]
fn lsb_propagates_from_ctx() {
    let v = Parent::try_from([0xf0].as_ref()).unwrap();
    assert_eq!(v, Parent { left: Child::Full, right: Child::Empty });
    // actual on master: Parent { right: Full, left: Empty } (msb read)
}

[amboar]

Not really a comment of substance, but:

struct Parent { right: Child, left: Child }

it seems like an odd choice to put the variable called right on the left, and left on the right

[sachaw]

lol, good catch.

[sachaw]

hey, any change I get get this merged, I have some other PR's to file afterwards.