reftable: introduce macros to allocate arrays

Similar to the preceding commit, let's carry over macros to allocate
arrays with `REFTABLE_ALLOC_ARRAY()` and `REFTABLE_CALLOC_ARRAY()`. This
requires us to change the signature of `reftable_calloc()`, which only
takes a single argument right now and thus puts the burden on the caller
to calculate the final array's size. This is a net improvement though as
it means that we can now provide proper overflow checks when multiplying
the array size with the member size.

Convert callsites of `reftable_calloc()` to the new signature and start
using the new macros where possible.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/publicbasics.c

@@ -37,8 +37,9 @@ void reftable_free(void *p)
 		free(p);
 }
 
-void *reftable_calloc(size_t sz)
+void *reftable_calloc(size_t nelem, size_t elsize)
 {
+	size_t sz = st_mult(nelem, elsize);
 	void *p = reftable_malloc(sz);
 	memset(p, 0, sz);
 	return p;