Reorganise Pancake directory

The `pancake/` subdirectory looks disorganised. Currently there are 22 files in the top-level directory, including the Pancake syntax definition, IR syntax definitions, backend pass definitions and static checks, among other things. And for some ungodly reason there's still a `temp/` directory with old cruft from `timeLang`? This is too much noise, and drowns out things we want people to find easily, such as `NEWS.md`.

This issue is to organise the files better. This could be done in many ways and I don't have a strong preference, but at the very least I think all compiler passes and IR definitions should go in subdirectories. Maybe there should be a subdirectory called `pancake/compiler/`?

Start by inventorying the 22 files in pancake/ and checking the old temp/ directory, with NEWS.md as an example of a file users should find easily. Group the compiler passes and IR definitions into subdirectories, remove obsolete timeLang cruft if confirmed, and consider whether pancake/compiler/ fits; done means the top-level directory is less noisy without breaking references.

Written by the indexing model from the issue text.