Add bitmaps-3.1.0

[nnethercote]

The final new benchmark I intend to add. I think it's a good choice because it stresses trait handling in very interesting ways, and in ways that are quite similar to nalgebra, which is another interesting case. However, bitmaps is much smaller than nalgebra, so it doesn't eat into our CI budget nearly as much.

End to end compile times:

- check: 1.35s
- debug: 1.35s
- opt: 1.50s

Final crate compile times vs helloworld:

Benchmark	Profile	Scenario	% Change	Significance Factor?	Test	Test
bitmaps-3.1.0	check	full	0.00%	-	0.963	0.963
bitmaps-3.1.0	debug	full	0.00%	-	0.957	0.957
bitmaps-3.1.0	opt	full	0.00%	-	1.078	1.078
helloworld	check	full	0.00%	-	0.23	0.23
helloworld	debug	full	0.00%	-	0.24	0.24
helloworld	opt	full	0.00%	-	0.218	0.218

[lqd]

It's indeed a great find. 40% of the compilation is in specialization_graph_of and evaluate_obligation, which is close to nalgebra as you said (although in different proportions) but 20x faster to build.