Removed fixed_stack_segment attributes (Rust uses large stacks by default now)
Refactor the code to prepare for concurrency

This rearranges the code a bit.  Dispatch is moved from request.rs to
session.rs, while code the reads and writes from the channel (which was
distributed between request.rs and session.rs before) now all resides in
channel.rs.  The channel hidden behind a "port" and a "chan" named
for (but not implemented with) Rust's standard Port and Chan.
Allow concurrent filesystem operations

This changes the signature of the operations to allow concurrent
operations, and adds an example that demonstrates their use.

Closes #1.