Merge branch 'fast-mr-experiments' into 'master'

Fast mr experiments

Supports fast evaluation of symbolic step matrices via pymbolic's compiler (see code [here]( https://gitlab.tiker.net/inducer/leap/blob/e4e7219fd15a3648df3bfc26218501cce6e801e0/examples/stability/fast_eval.py))

See merge request !1