Add shift left/right by constant amount

[whitequark]

This has several benefits:

• Parity with Add rotate left/right by constant amount #339;
• Unlike <</>>, shift_left/shift_right can accept a signed argument without it being a Python compatibility hazard;
• Unlike <</>>, shift_left/shift_right can return a result that has the exact bit width, as opposed to the most conservative overapproximation. I expect that at some point it will be requested that width inference for <</>> be special-cased for constant amounts, and this is a good way to preempt that.

Note that unlike rotate_{left,right}, shift_{left,right} should return a signed result if the LHS is signed.

[whitequark]

Documented in f24f1b0.