Rename StrBuf to String

[alexcrichton]

Tracking issue for rust-lang/rfcs#60

Nominating.

[richo]

I tried asking on IRC, hoping to get this out tonight because it looked achievable.

What's the plan for the String trait as well as the existing String type. I feel like limiting the visibility of the string type is probably fine, but the String trait is necessary for formatters.

[pnkfelix]

P-backcompat-libs, 1.0.