Fix various CCD and broad-phase related bugs

[sebcrozet]

fix some issues discovered while updating bevy_rapier.

• CollisionPipeline::step no longer trips a debug assertion when a moving body touches another
  collider.
• A zero-length step no longer corrupts the simulation: it only applies the user changes and runs
  the collision detection.
• Enabling or disabling an impulse joint now updates the islands of its bodies.
• Multibody DoF couplings and disabled self-contacts are no longer lost when multibodies merge or split.
• PhysicsPipeline::counters now fills the contact pair, contact, and constraint counts.
• rapier3d-urdf: the fixed children of a squeezed empty link now stay rigidly attached together, and
  the joint taking over the removed link's joint keeps its original pivot.
• 2D soft-body cluster proxies now follow counterclockwise rotations of their particles.
• The rigid colliders attached to soft-body cluster proxies no longer lag one step behind their proxy.
• The broad-phase AABBs of soft-body surfaces now match their end-of-step geometry.
• When the CCD splits a step, the end-of-step soft-body motion margin and soft-CCD AABBs now cover the
  full next step instead of the last CCD pass.
• The CCD now also splits the first step of a fast body at its impact.
• The soft-body contact history driving the extra substeps now counts steps instead of CCD passes.
• The CCD fast-body check now divides the applied forces by the body's mass, so heavy bodies under
  gravity are no longer flagged as fast.