Provide new multiplication methods for Diagonal matrices

- Provides A_mul_B!, At_mul_B! and Ac_mul_B! for A::Diagonal
- Renames Ac_mul_B and A_mul_Bc to Ac_mul_B! and A_mul_Bc! for A::Triangular (to reflect mutation of B)
- Provides A_mul_B! for A::Triangular