Cache codegen result in freeze() (#56)

* adds utils for normalizing an array expr

Co-authored-by: Andreas Kloeckner <andreask@illinois.edu>

* PytatoArrayContext: hold a cache of frozen arrays to programs

Co-authored-by: Matthias Diener <mdiener@illinois.edu>

* bugfix: change order of pt.make_placeholder

* Clarify hashing -> caching in normalization function docstring

Co-authored-by: Kaushik Kulkarni <kaushikcfd@gmail.com>
Co-authored-by: Andreas Kloeckner <andreask@illinois.edu>
Co-authored-by: Andreas Klöckner <inform@tiker.net>