investigate providing core library functionality in a no_std environment

[sdemos]

If enough of the heap allocation is removed from the core data structures and functions of this library, it should be possible to provide a no_std feature flag, so libraries and executables that are operating in a no_std environment can still use this library. It will likely remove some more complicated convenience functionality.

It's possible that this isn't a library that anyone will want to use in a no_std environment, but I think that if the work in #17 #18 #19 is complete, it should be relatively easy to allow this, so we might as well investigate it.