spec: SHIFT chip updates

[RobinJadoul]

• The definition of shifted looks to have an off-by-one bug in the contribution of extension.
• SHIFT-C2 doesn't work when shift = 0; the suggested solution would be to let SHIFT-C2 become AND_BYTE[bit_shift; 256 - 16 * zbs - shift, 15]. The interaction of this with SHIFT-C3 looks to be fine
• It's likely possible to shave another column by recovering one of the limb_shift columns from the other three