Transform: rename stream_transformation to to_stream_fn
Stream docs: explicitly include stdlib elements, organize into sections

Also improved documentation of several elements. This addresses #64 but
a bit more polishing is needed.
Streamable: move to its own module

This follows Core style.
Streamable: rename type streamable to t

Using distinct type names for modules intended to be included is not
necessary since OCaml 3.12, which introduced destructive substitution
in module constraints.