PytatoPyOpenCLArrayContext: use SVM allocator if available, limit arg size for GPUs (#189)

* LazilyPyOpenCLCompilingFunctionCaller: limit arg size for GPUs

* move limit

* also check for SVM presence

* get_target()

* memoize get_target

* UNDO BEFORE MERGE: use dev branches

* Hackety hack: SVM detection in actx constructor

* check whether passed allocator supports SVM

* undo loopy branch

* implement it for the base class

* subclass LoopyPyOpenCLTarget

* set actual limit

* undo pytato branch

* remove unused argument

* add type annotations

* add logging

* Refactor arg size passing to put less logic in the target

* flake8

* add a test

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