Clean out more strbuf references

[alexcrichton]

• Removes format_strbuf!()

[huonw]

I'm not keen on the Add impl for string. It seems to easily encourage inefficient/overallocating code since it doesn't reuse the allocation of its argument, meaning s = s + a is horrible (especially in a loop). (And I relatively often see code approximately equivalent to that.)

I would be very interested in waiting a little longer to see if not having it is just too unbearable.

(Also this addition on strings isn't even commutative, i.e. it flies in the face of mathematical convention for the + operator. :P )

[sfackler]

Legit travis failures.

I'd also prefer to leave the Add impl out for now.

[alexcrichton]

Moved Add to #14482