Updates

Update programs describe how one active branch rewrites x into x'.

The user-facing builder is nkdsl.Update, but most code starts from the module-level helpers.

• identity()
• shift(site_ref, delta)
• hop(src, dst, amount=1)
• shift_mod(site_ref, delta)
• write(site_ref, value)
• swap(...) and permute(...)
• affine(...) and scatter(...)