GLib is discussing deprecating/removing it upstream [1] since it has
only limited uses. Next to that, it seems to bork stack traces here when
using ASAN (for which you also have to specify `G_SLICE=always-malloc`
and some other envvars too).

In other words, let's just get rid of using `GSlice` and call the
allocation APIs directly.

[1]: glib#1079