There are a lot of gbooleans in the private GTask struct, which seems a
bit wasteful. Use a bitfield to compress the struct a bit.

This reduces the size of the struct from 216 bytes to 168 bytes on my
64-bit machine.

One of the fields needs to remain separate, since it’s used from a
TRACE() macro which calls typeof() on it.

Signed-off-by: Philip Withnall <withnall@endlessm.com>