It concatenates multiple string-like entities in one go and gives away
an sstring. It does at most one allocation for the final sstring and
one copy per each string. Works with heterogenous arguments, both
sstrings and constant strings are supported, string_views are planned.
It's required for instantiating a sstring with the constructor
basic_sstring(initialized_later, size_t size).
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>