Optimize string-based connect() vis-a-vis PMF-style connect()

In the eternal debate whether string-based or PMF-based QObject::connect() is "better", dfaure_kdab offered the argument that the latter are faster than the former.

tst_bench_qobject confirms this (not posting benchmark results, because they're done with an ubsan/asan build), but, at least for normalized signatures, this is kinda surprising, because the PMF-connect needs to allocate an (additional) QSlotObject instance, while string-based connect() merely needs to parse (compare, really) a (relatively short) string.

Investigate why string-based connect is so slow and try to optimize it, starting with de-pessimisation.