v0.13.x: `vec` matrix operations performance improvements

• This is a small but mighty release, in which vec's representation of a Matrix got changed to 2x3 matrix to save some time and memory. Some functions in vec were also rewritten to reduce function calls. Thanks @sritchie for this change (#92)!
• Mafs now has visual testing for all examples on the docs site. This greatly increases the confidence in shipping changes to Mafs.