Unicode identity matrix symbol

This adds a [double-struck 1](http://www.fileformat.info/info/unicode/char/1d7d9/index.htm) as an alternative to `I` for the constant representing the identity matrix. While this is obviously nice, and there are the precedents of `π`, `φ` etc., I can think of two possible reasons not to have it:

1. I may not be aware of a more appropriate Unicode symbol, this being the only suitable one I found browsing around
2. Github doesn't currently allow it in comments, which could in principle create minor hassles. At any rate, this is the reason why I didn't touch how UniformScaling matrices are shown.