fix minor stepper damage caused by a9f46ade9ea16eba7507
simplify stepper and fix unevaluated id bug
tiny test for letrec
make plot tests more drdr friendly

This avoids a race-condition when one test deletes the file
that another creates and also avoids cluttering the current
directory.
Unbreak TR contracted build.
Add loop function in cgen/arr in preparation for refactoring.
Move common checks out of loop in cgen/arr.
Switch input from arities to sequences.
Split out loop and rename it cgen/seq.
Make cgen/seq take its args in the correct order.
Cleanup some cases of cgen/seq.
Make value inference just delegate to sequence logic.