despite not being a b_object, the b_stream interface can now detect when
a b_bstr instance has been passed as a stream parameter and use the correct
functions to handle writing to it.
this allows any function that expects a b_stream parameter to be called
with a b_bstr instance instead, allowing complex stream-based I/O operations
to be directed towards bounded character arrays with no heap allocation
required.
b_stream can now read/write utf-8 encoded data, and will do so unless
the stream is in binary mode.
b_stream will also report an error it encounters invalid utf-8 data
(unless binary mode is enabled).
b_stream represents a character-based I/O stream. it is an abstract interface
that can be implemented by any other object, allowing the object to be
interacted with via the b_stream api.
