Drop use of GLib GSlice allocator

There's little evidence that the GSlice allocator still provides any advantages over the standard glibc allocator or Windows allocator in 2023. There is strong evidence that it causes memory fragmentation for standard GStreamer workloads such as receiving rtsp streams, however.

See #291 (closed) for discussion and details.

This MR is still in draft, as it appears to cause some test failures (clock/systemclock related), and some fix-ups need to be merged back into their respective commits.

The commits have been split up as to facilitate bisecting or strategic reverts in case of hard to track down problems later.