drt: Use boost flat hashmap when beneficial

[sgizler]

• Bump boost to get flat hashtables
• Replace std::set indexed by pair of Points with boost boost::unordered_flat_set
• Replace std::map indexed by Point3D with boost::unordered_flat_map
• Replace std::unordered_map with boost::unordered_flat_map (this was the single case, in which DRT already used hashtable rather than search tree)

Benchmarks of drt:

ibex, PDK: asap7

Branch	Min [s]	Max [s]	Mean [s]	Relative mean	Median [s]	Relative median
boost-flat-hashmap-opt	286.27	287.44	286.77 ± 0.35	1.00 ± 0.00	286.73	1.00
baseline	290.79	292.14	291.41 ± 0.41	1.02 ± 0.00	291.35	1.02
boost-1.86	290.49	292.40	291.36 ± 0.54	1.02 ± 0.00	291.26	1.02

ibex, PDK: nangate45

Branch	Min [s]	Max [s]	Mean [s]	Relative mean	Median [s]	Relative median
boost-flat-hashmap-opt	87.66	88.38	88.00 ± 0.17	1.00 ± 0.00	88.00	1.00
baseline	89.15	89.83	89.39 ± 0.22	1.02 ± 0.00	89.31	1.01
boost-1.86	89.11	89.51	89.33 ± 0.12	1.02 ± 0.00	89.31	1.01

black_parrot, PDK: nangate45

Branch	Min [s]	Max [s]	Mean [s]	Relative mean	Median [s]	Relative median
boost-flat-hashmap-opt	1035.30	1039.40	1036.90 ± 1.01	1.00 ± 0.00	1036.85	1.00
baseline	1056.01	1057.20	1056.48 ± 0.33	1.02 ± 0.00	1056.44	1.02
boost-1.86	1057.09	1058.94	1057.84 ± 0.54	1.02 ± 0.00	1057.72	1.02

[maliberty]

Please fix the missing DCO (see the details like).

[vvbandeira]

Please also merge or rebase the master branch into your branch to get the latest changes to CI so pr-head can build.