call_loopy: perform shape inference and infer loopy size parameters (#94)

* mypy show error codes

Co-authored-by: Andreas Kloeckner <inform@tiker.net>

* gets rid of unnecessary type-ignore

* dim_to_index_lambda_components: makes interface more convenient

caller can pass None for variable name generator, in which the routine will
create a unique name generator of its own.

* call_loopy: shape inference implementation

Co-authored-by: Andreas Kloeckner <inform@tiker.net>

* test call_loopy with shape inference

* move SizeParamGatherer to pytato.transform

* starts being less invasive about overwriting dtypes

* Add, clean up *Gatherer docs

Co-authored-by: Andreas Kloeckner <inform@tiker.net>