A new PEP for infix matrix multiplication

This is now PEP 465, and a properly rendered version can be found here [here](http://legacy.python.org/dev/peps/pep-0465).

A poorly rendered version of the latest draft can be found [here](https://github.com/njsmith/numpy/blob/matmul-pep/doc/neps/return-of-revenge-of-matmul-pep.rst).

Let's use this PR as a central place to discuss this draft PEP for adding an infix matrix multiplication operator to numpy. The PR format is nice in that it allows line by line comments, updating, etc.

Hopefully this goes without saying, but just in case: **We especially welcome comments about how the proposal will work for non-numpy projects (or could work, with fixes)**; this proposal started in the numpy community but the idea here is to build a consensus about how to make Python better for *all* projects that care about linear algebra.

Some possible points of discussion:
* Does this in fact seem like a good idea?
* Do people agree with the points where I claim "there is consensus that..."? (You can search on "consensus")
* Any suggestions on making the argument stronger and clearer?