std::to_string() can also convert a floating-point argument to string,
using the "%f" printf format (see
http://en.cppreference.com/w/cpp/string/basic_string/to_string )
So add this support to our to_sstring() as well.
Note that if you want to use a different format, e.g., "%g", you can,
by using the to_sstring_sprintf function, for example
to_sstring_sprintf(12345678.9, "%g")
results in "1.23457e+07".
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
5b071fd224