Compute inverse of Cholesky with a single solve

[devmotion]

Alternative to #1258 that instead of averaging out the errors of the two solves computes the inverse with a single solve and a symmetric matrix multiplication.

Checks of symmetry in the tests pass locally, and the example in the tests is even slightly faster:

This PR

julia> using StaticArrays, LinearAlgebra, Chairmarks

julia> @be cholesky((x -> x * x' + I)(@SMatrix(randn(3,3)))) inv
Benchmark: 3702 samples with 4741 evaluations
 min    4.561 ns
 median 5.018 ns
 mean   5.419 ns
 max    11.680 ns

master

julia> using StaticArrays, LinearAlgebra, Chairmarks

julia> @be cholesky((x -> x * x' + I)(@SMatrix(randn(3,3)))) inv
Benchmark: 3756 samples with 3276 evaluations
 min    6.983 ns
 median 7.326 ns
 mean   7.712 ns
 max    53.406 ns

[mateuszbaran]

I think this is a good idea.