More microoptimizations

[pleroy]

On the C++ side:

1. Replace the std containers with absl containers whenever possible. This saves maybe 5% of the CPU.
2. Minor code cleanups to use more modern C++.

On the C# side:

1. Use SetPositionAndRotation to change the transforms. This is supposed to be faster but I guess that it has no effect when the physics is not auto-syncing.
2. Use TryGetValue instead of ContainsKey + Item[], this saves a lookup.
3. Rewrite the inner loop that looks at all the PartModules of a Part to use a vanilla iteration instead of Linq. This saves about 10% of the CPU.

At this point we are reaching diminishing returns: Principia is using about 25% of the time of a frame, so any further optimization is going to be essentially invisible.

Fix #3230.