Bevy: NPhysics Backend

[cart]

Once we implement #83, we will need a backend for it. I think nphysics is a good choice as it is pure rust, has a decent feature set, and supports both 2d and 3d physics.

[ncallaway]

Consider also Rapier (https://rapier.rs), which was announced very recently and sounds like it's now the primary focus of the nphysics developer (which nphysics now listed as passively maintained https://www.dimforge.com/).

[Ratysz]

@ncallaway This seems to be already happening, see https://github.com/bevyengine/bevy/issues/83#issuecomment-677897740 and on.

[cart]

Given that there is already an official bevy_rapier physics plugin, I'm comfortable closing this.