Add 'compact-display' feature that prints vectors and matrices compactly

[anderslanglands]

I use nalgebra for writing a renderer and other computer-graphics applications. I often print out vector and matrix data for debugging and information purposes, but the default Display implementation is extremely large, especially when printing an array of values.

This PR adds a feature, "compact-display" that switches to an alternate implementation which prints vectors and matrices as a parenthesized list of rows to save on space.

[Ralith]

A simpler approach is to use debug formatting on matrix.as_slice(), if you can stand column-major presentation.

[anderslanglands]

Hadn't thought of that, but it does make it a bit ugly and noisy at the call site.

[Andlon]

Closing this old PR due to lack of activity/interest. Thanks for the interest in nalgebra!