This lets us make a lot of other stuff const, allows the compiler to generate (slightly) better code, and will make me get slightly fewer patches from folks who stick mutable stuff into or_options_t. const: because not every input is an output!
Also remove some #if 0'd code from the unit tests for buffers. The code was killed in e6794e5808 (5 years ago), and is now broken anyways.
e6794e5808