WIP: Mat3x4

[emilk]

Part of #25

This was more work than I thought (it always is). I can't say I fully grok how all the different traits fit together, but I tried to copy the basic structure of the 4x4 types. I still have some hard-to-decipher errors.

TODO:

• Get it to compile
• Finish up the TODO:s an commented code
• Write tests
• Write benchmarks

[bitshifter]

Thanks for that, I haven't had a chance to have a proper look yet, hopefully I'll find some time over the long weekend. It might pay to flesh things using the public types and look at the core stuff later. It is a bit complicated unfortunately and I haven't had to think about how to deal with new types yet.

[emilk]

No hurry - I (probably) won't continue work on this until Tuesday. And yeah, I should have started with

struct Mat3x4 {
    x_row: Vec4,
    y_row: Vec4,
    z_row: Vec4,
}

I might make another PR with that so we can try that out and do benchmarks etc.

[bitshifter]

These should be $vec3

[emilk]

Closed in favor of #157