Need while loops

[inducer]

To implement chemistry. An additional problem with this is that there isn't a way to lower those to Loopy.

cc @kaushikcfd

[wence-]

I vaguely recall speaking to Paul and Christian Lengauer about while loops in the polyhedral model: there is some way to handle them ISTR, but don't remember what it is.

[inducer]

Given that functions are (starting to be) a thing in pytato, maybe we could avoid adding all sorts of control structures and just allow tail calls? (came up in a discussion with @nkoskelo)