Refactor content of axiom/stream directory

[rpaquay]

axiom/stream contains too much stuff right now

• utility classes (e.g. queue.js)
• low-level stream interfaces (e.g. ReadableStream)
• various stream implementation (e.g. MemoryBufferStream)
• stuff related to remote file system (e.g. stub*.js, skeleton*.js, transport.js, channel.js)

Given the inventory above, it seems we should move these files into (at least) 4 distinct directories...

[rpaquay]

We might also use this opportunity to rename classes.

• StubFileSystem => RemoteFileSystem
• SekeletonFilesystem => RemoteFileSystemProxy

[ussuri]

+1. Also, should streams even be under the fs directory? They don't have much to do with file systems, do they?