Function expand_invert_mod2k_step 

const fn expand_invert_mod2k_step(
    a: &Odd<UintRef>,
    buf: &mut UintRef,
    buf_init_len: usize,
    scratch: (&mut UintRef, &mut UintRef),
)

One step of the Hensel quadratic modular inverse calculation, doubling the width of the inverted output, and wrapping at capacity of buf.