treewide: rename assumptions to kr_assert() / kr_fails_assert()

treewide: rename assumptions to kr_assert() / kr_fails_assert()

To (hopefully) improve readability, rename the typical macro usage of:

    if (!kr_assume(x)) y; // to
    if (kr_fails_assert(x)) y;

As a convenience, replace the assert without a return value to a more
simple version:

    (void)!kr_assume(x);  // becomes
    kr_assert(x);