refactor(mat!): convert mat! into a syntax extension

[japaric]

The new mat! expansion does much less work at runtime.

If you were using the mat! macro, you'll have to change your crate linking from:

#[phase(link, plugin)]
extern crate linalg;

to

extern crate linalg;
#[phase(plugin)]
extern crate linalg_macros;

[breaking-change]