Handle very large `.emb` files. (#215) This change switches parse tree handling to use iteration (with an explicit stack) instead of recursion, which: * Allows large (>~1000 entity) `.emb` files to be formatted. * Allows very large (>~16k entity) `.emb` files to be compiled. The difference in sizes in the previous code was due to `module_ir.py` hackily increasing the recursion limit: while this more or less worked, it was a little dangerous (it ran the risk of blowing out the C stack, depending on platform) and only increased the limit. This change removes the limit entirely (at least, up to the available memory on the system).