Implementation of fmt::prepare() function and features around it.

Thanks to Howard Hinnant.

The iterator class itself and its helpers 'to_pointer' are removed after reformulating the formatting loop in terms of a 'basic_string_view<Char>::iterator'.
Signed-off-by: Daniela Engert <dani@ngrt.de>

scoped_ptr was removed in with gtest google/googletest@e857f9cdd998136b9aad634272301f5b2d0476ea.

* Closes #993

* Fix sign conversion warnings

* Ignore unused local typedefs

* Remove cast reported as useless on GCC

* Remove warning on unused variable

* Enable conversion warning checking for pedantic build

* Fix sign-conversion warnings in headers