Simplify appending SBuf to String (#2108)

Simplify appending SBuf to String (#2108)

There is still a lot of code that uses legacy String class, and an
increasing amount of code that has to append modern SBufs to Strings.

SBuf handles NUL bytes better than String. With respect to handling NUL
bytes, the new append() method is as (un)safe as the existing explicit
size-based one (that callers tend to use prior to these changes anyway).