Implement abs for size and vector types.

[nical]

Fixes #231.

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This change is 

[nical]

r? @kvark

it'd be nice to have a method for size that would clamp all negative components to zero, since that's the other common interpretation of negative sizes (depending on the context). I don't know how that method would be called.

[kvark]

Looks fine by me. Have you considered implementing Signed for TypedSize2D and friends instead of adding free-standing methods?

[nical]

I was on the edge and wasn't sure whether it would be controversial. I can definitely do it if you have a preference towards it. That means implementing:

    fn abs(&self) -> Self;
    fn abs_sub(&self, other: &Self) -> Self;
    fn signum(&self) -> Self;
    fn is_positive(&self) -> bool;
    fn is_negative(&self) -> bool;

I am not sure what the most sensible thing is for is_negative between:

• returning true if all components are negative,
• returning true if at least one component is negative.

[kvark]

Oh, I assumed at least some of those are not required to implement. Nevermind then :)
Also, it looks like the changes are non-breaking, which is good.
@bors-servo r+

[bors-servo]

📌 Commit 5096fd4 has been approved by kvark

[bors-servo]

⌛ Testing commit 5096fd4 with merge 22a5145...

[bors-servo]

☀️ Test successful - status-travis
Approved by: kvark
Pushing 22a5145 to master...