Forward declaration necessary when used stand-alone.
Add memory dump capability to leak detector

Rather than printf the contents of arbitrary memory into the leak
detection report (which breaks UT on various compilers/platforms/
optimization levels, and probably JUnit output), print out a
hexdump-like view of the memory, with printable ASCII where
possible.
Fix initialization of SimpleStringBuffer to empty string

Previously, the state of buffer_ was undefined at initialization.
This may cause a problem with tests that depend on an 'untouched'
SimpleStringBuffer returning an empty C string via toString().