Add directives for accessing and remapping streams

[csnover]

Initially I used reader/writer/map_reader/map_writer instead of just stream, but since a Read + Write + Seek is compatible with either, using stream/map_stream in the directive makes it possible to use #[brw] in some cases, where otherwise it would be impossible.

Unresolved questions:

1. Should there be a try_map_stream too?
2. Symmetry with Add helper attribute macros for writing free function parsers/writers #171: (a) in terms of putting the stream into the #[binread]/#[binwrite]/#[binrw] attribute instead of using a directive; (b) should Add helper attribute macros for writing free function parsers/writers #171 also just use the word stream instead of separating reader/writer?