When a user defines an output operator for std::basic_ostream and
a pointer to a user-defined type, that operator use to be ignored
because of the implicit cast to const void* that happened when
operator<< for formatting_ostream or record_ostream was called.
We now forward the pointer type to the operator<< for std::ostream,
so it is either cast then or user's operator<< is picked.
Fixes https://github.com/boostorg/log/issues/84.
Explicit output operators allow to enable character code conversion, if
one is needed, on string view output. This is in line with the existing
support for other string types.
