Since assert() can be compiled out, and users will vehemently argue for
their right to do so, just explicitly check everything we absolutely
100% want to crash on instead of blindly continuing.

For convenience, a helper function is introduced for this (pl_assert),
which is defined as assert in the common case (debugging builds), but
otherwise gets compiled to an if () abort();

This should prevent all serious cases of undefined behavior from
occurring without requiring users to build debugging builds.