v0.2.0

Added constexpr to trivial cases such as functions, class constructors, and methods that don't use index operators (mostly for vectors). I am unable to use constexpr in more places because of the inability to mark accessors (at() and operator[]) as constexpr due to the inability to arbitrarily switch active union members. If I find a workaround for this that is not too invasive then I can look into it.