'v_a_vT' is slow and named wrong

[bvdmitri]

The function v_a_vT does not do what the name suggests, instead of v * a * v' it does v * v' * a. It works OK, because in the only context we use this function a is a number, so it commutes, but the name is misleading.

Moreover, a naive implementation of v * v' * a outperforms our version by a factor of 4:

v = ones(3)

@btime RxInfer.ReactiveMP.v_a_vT($v, 1)

102.908 ns (1 allocation: 128 bytes)
3×3 Matrix{Float64}:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0

foo(v, a) = v * v' * a

@btime foo($v, 1)

26.674 ns (1 allocation: 128 bytes)
3×3 Matrix{Float64}:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0