This allows us to provide machine-readable listings through the
XMLReporter and it will also allow users to define their own listing
format that does whatever their own tools need.
Previously it returned the sum of listed things because ???. This
was completely useless and in many ways actively counterproductive
because of the success/failure conventions around exit codes.
Closes#1410
Noticed that the code was originally concatenating strings just to
then append the result to another string. Now it does not create
temporaries and also preallocates the string buffer.
ReusableStringStream holds a std::ostringstream internally, but only exposes the ostream interface.
It caches a pool of ostringstreams in a vector which is currently global, but will be made thread-local.
Altogether this should enable both runtime and compile-time benefits. although more work is needed to realise the compile time opportunities.
