fix aliasing UB, layout UB, and incorrect initialization UB

[Morgane55440]

the get_disjoint_mut-likes are aliasing UB under the current memory model.
for more info, see the internal code of get_disjoint_mut and the relevant history.

as_vector on RigidBodyVelocity is UB because it is not #[repr(C)]

reset_buffer can cause incorrect initialization UB because it initializes len bytes, but it says it has initialized len * size_of:::<T> bytes.

the get_disjoint_mut-likes have been checked to result in the same assembly after fix :

• https://godbolt.org/z/5TTfrs4zP
• https://godbolt.org/z/Tdbh75PT9

the asm for reset_buffer is slightly different, but it still uses memset which is the important part.