Improve ROT_TWO support

[meadori]

This partially fixes #88 by improving how code for ROT_TWO
is generated. In particular, optimized locals are used for
integers and floats. A similar fix will be made for ROT_THREE
in a following patch.

[msftclas]

Hi @meadori, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

In order for us to evaluate and accept your PR, we ask that you sign a contribution license agreement. It's all electronic and will take just minutes. I promise there's no faxing. https://cla.microsoft.com.

TTYL, MSBOT;

[msftclas]

@meadori, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, MSBOT;

[brettcannon]

I just wanted to quickly say thanks for the PR, @meadori ! I don't know how long we will take to review the PR but I promise we will get to it (both @DinoV and I have presentations to prep for next week).

[DinoV]

Mostly looks great, but what happens with mixed variables, e.g:
x = 1.0
y = 'abc'
x,y = (y, x)

It seems like we should be marking them as escaping if they differ?

[meadori]

Good point. I updated the code to escape if the types differ and I added a few more test cases.

[DinoV]

Thanks for your contribution! That's the first significant contribution we've received!