[d-torrance]

basisParseFromString, the test helper that parses a ring and a basis from a
string, returned only the basis:

  auto p = MathicIO<>().readRing(true, in);
  auto& ring = *p.first.release(); // todo: fix leak
  return make_unique<Basis>(MathicIO<>().readBasis(ring, false, in));

A Basis refers to its ring without owning it, so once the unique_ptr was
released nothing owned the ring. That todo dates from bbb09cb in 2013.

• IO.ideal and Ideal.readwrite leaked the ring. That was the whole of
  the suite's ASan leak report: 1216 byte(s) leaked in 8 allocation(s).
• Poly.lead avoided the leak by taking ownership itself, through the const
  pointer getPolyRing() returns -- and declared that owner after the basis,
  so the ring was destroyed before the basis that refers to it.

The fix

It is now ringAndBasisFromString, returning both:

std::pair<std::unique_ptr<PolyRing>, std::unique_ptr<Basis>>
ringAndBasisFromString(std::string str);

The three tests bind them with auto [ring, I] = ..., so the ring lives exactly
as long as the basis and, since std::pair destroys its members in reverse, is
destroyed after it. Poly.lead drops its workaround.

Why rename it, not just change its return type

C++ names do not encode the return type. Changing only the return type would
have kept the exported symbol mgb::basisParseFromString(std::string), so a
binary built against the old header would still link and then read one pointer
where two were written. With the old name gone, such a binary fails to load
with an undefined symbol instead.

io-util.hpp is installed and the function is exported from libmathicgb.so,
so this is an ABI change to note for the release. Nothing outside this
repository is known to call it: its only reverse dependency packaged in Ubuntu
is Macaulay2, which does not, and a GitHub code search finds only copies of
mathicgb.

Verification

• 254/254 in cmake Release and Debug, GCC 13.3, no build warnings.
• The full suite now runs clean under ASan -- no leak report at all, where
  master reports 1216 bytes in 8 allocations.
• clang 18 with -Wall -Wextra raises nothing new in the changed files.

No autotools distcheck run locally; the commit touches no build system and CI
covers it.

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