experimenting with optimizations

[Equanox]

Performance optimizations:

• Use ints to reference between tasks instead of a hash map.
• Use play() less often.
• Create the build graph in one walk.
• Ignore duplicate target check (till optimized). Perfomance verify duplicate targets #286
• Use env cash to BuildNixDep less often. performance: nix dependencies #287

Brings down raw bob overhead from ~3s to 720ms on a repository containing 5000 Golang projects.