[PATCH] String API re-implementation using SBuf

This patch implements Alex's suggestion for step 1 of String removal by ...

Replace String class internals with an SBuf.

This method allows all String code to be converted quickly to SBuf
without so much worry about correct API conversion elsewhere around the
code since the wrapper provides exemplar cut-n-paste code to work from.

Addition of a SBuf constructor and toSBuf() accessor to String allows
code using String to be converted directly to the SBuf API in smaller
portions later. These obsolete the SBuf API converters to String and
help reduce the SBuf API.

Amos