bool is dangerous in a time-safe context, because C compilers might
insert a control flow divergence to implement the implicit
normalisation of nonzero integers to 1 when you assign to a bool.
Everywhere else time-safe, I avoid using it; but smemeq has been an
exception until now, because the response to smemeq returning failure
was to do an obvious protocol-level divergence _anyway_ (like
disconnecting due to MAC mismatch).
But I'm about to want to use smemeq in a context where I use the
result _subtly_ and don't want to give away what it is, so now it's
time to get rid of that bool and have smemeq return unsigned.
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