matrix arithmetic operations

faer matrices implement most of the arithmetic operators, so two matrices can be added simply by writing &a + &b, the result of the expression is a faer::SparseColMat or faer::SparseRowMat, which allows simple chaining of operations (e.g. (&a - &b) * &c), although at the cost of allocating temporary matrices.

for more complicated use cases, users are encouraged to preallocate the storage for the temporaries with the corresponding sparsity structure and use faer::sparse::ops::binary_op_assign_into or faer::modules::core::ternary_op_assign_into.