This formalises my occasional habit of using a single malloc to make a
block that contains a header structure and a data buffer that a field
of the structure will point to, allowing it to be freed in one go
later. Previously I had to do this by hand, losing the type-checking
advantages of snew; now I've written an snew-style macro to do the
job, plus an accessor macro to cleanly get the auxiliary buffer
pointer afterwards, and switched existing instances of the pattern
over to using that.
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