Iostreams: Additional Filters and Devices

1. Filters and Devices based on running an external process, as in libexecstream
2. Base64 encoder and decoder
3. Hex encoder and decoder
4. grep filter
5. A seekable_filter_adapter that provides an implementation of seek() when the user has defined read() and write(). seek() would work by checking whether the offset is relative to the beginning of the stream, and if so, whether it corresponds to a previously saved offset. If so, it fetches the stored offset in the unfiltered stream and performs a seek on the downstream device. This would solve the problem raised by #484