Define NDEBUG actively•••`NDEBUG` is not defined by the standard library or the compiler by
default, so make sure it's defined when building non-debug builds.
Elvis Pranskevichus
2020-04-22
A bunch of fixes•••* Fix #26: `ifdef NDEBUG` should be `ifndef NDEBUG`
* More tests for #24
* Add a `DEBUG_IMMUTABLES` env var to build debug builds