Move MarkBag* funcs into GCs so they can inline MarkBag

[ChrisJefferson]

While tracking down a slowness, I found that splitting functions involved in marking bags between different files created a significant slowdown in gasman. This patch pulls these functions into a header which should be included into each GC. I'm not totally happy with how this is done, but it feels like the best way to do things without duplicating code.

The ".inc" extension is something I've used in other systems for headers which must only be included in one file. I'm open to alternative suggestions. Another (horrible) option would be to put this stuff into gasman.h, guarded by a macro (IN_GC?), which would be #defined in each GC file...

On master:

gap> l := List([1..10000], x -> ListWithIdenticalEntries(10000, 0));; # make some stuff
gap> GASMAN("collect");
gap> time;
649
gap> GASMAN("collect");
gap> time;
501
gap> GASMAN("collect");
gap> time;
453

With this patch:

gap> l := List([1..10000], x -> ListWithIdenticalEntries(10000, 0));; # make some stuff
gap> GASMAN("collect");
gap> time;
396
gap> GASMAN("collect");
gap> time;
273
gap> GASMAN("collect");
gap> time;
295

[ChrisJefferson]

I am aware this needs some cleaning up, but I'm interested on opinions on the general layout (in particular having the Inc file at all)

[fingolfin]

Wow, that's massive. And my fault, I guess (never expect inlining to be so vital in this case, but I should have measured back then sigh).

I am not "hot" for those .inc files, but I don't really see a better alternative. Of course we could also just return to the old way and duplicate the code to each of the three GC implementations. Honestly, both of these options seem equally appealing to me.

[fingolfin]

I just checked, and luckily, this regression is not present in GAP 4.10, but of course it is present in the (as of yet unreleased) GAP 4.11. As such, I'd like to backport this to GAP 4.11, if possible (CC @alex-konovalov).

[coveralls]

Coverage increased (+0.0003%) to 84.728% when pulling 34eedc4 on ChrisJefferson:inline-mark into db5b45d on gap-system:master.

[fingolfin]

Backported in 8b3ec23