A crashing bug got mistakenly inserted because a new contributor (reasonably) assumed that the `halide_assert()` macro in our runtime code was like a C `assert()` (i.e., something that would vanish in optimized builds).

This is not the case; it is a check that happens in all build modes and always triggers an `abort()` if it fires. We should remove any ambiguity about it, so this proposes to rename it to somethingmore like the Google/Abseil-style CHECK() macro, to make it stand out more.

(We may want to do a followup to verify that all of the uses really are unrecoverable errors that aren't better handled by returning an error.)