Better infinities and abs.

[orlp]

This is a feature available in Pyth 5, but we might want to backport for the time being.

Four overloads:

• +) -> float('inf')
• -) -> -float('inf')
• +Z) -> abs(Z)
• -Z) -> -abs(Z)

This mean we can also remove infinity from the list of mathematical constants .n, and remove .a.

[isaacg1]

I added some other things to .a specifically vector length, so I'm going to keep it around. The other uses don't seem common enough to warrant this, so I'll leave it out for now.