make the conditional in bit shifts and rotates explicit

A backend can then assume that the bitwise shift or rotate instruction is given a positive (> 0) shift amount, and exposes the conditional to the optimiser.
trafo: simplify (>->) applied to identity
Chunked sequence execution using Vector' in interpreter (vect. of mapSeq and zipWithSeq still TODO)
Moved Vector' definition to Array/Lifted.hs
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