A new PEP for infix matrix multiplication

[Please don't merge]

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.

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")
* Are we confident how we want `@` to treat scalars? (Actually I'm not, I'll send a separate email to the list about this.)
* Any suggestions on making the argument stronger and clearer?