assert() is now replaced by assert_invariant() which has the same
prototype and (currently) behaves the same than assert().
<assert.h> should not be used anymore, and is replaced by
<utils/debug.h>, which is where assert_invariant() is found.
The main motivation for this is to be able to set a breakpoint on the
assertion as lldb doesn't handle abort() very well, and doesn't
permit to inspect the stack trace.
A secondary motivation is to be able (at some point) to enable
assertions without necessarily doing a debug build.
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