* Add handler_ptr test and increase coverage
* Add test for prepare_buffer
* Move is_call_possible tests to a .cpp file
* Tidy up docs and declarations
buffer_cat now determines if all of the buffer sequences in
the list of passed buffer sequences each have value types which
are convertible to mutable_buffer. The returned concatenated
sequence will be a MutableBufferSequence if all the passed
buffer sequences meet the requirements of MutableBufferSequence,
else the returned concatenated sequence will be a ConstBufferSequence.
Vinnie Falco