Clean up exports, use *Qid for sessions
Dont export R* types, make T*.Respond take the needed args.
Storing Qid pointers reduces the uglyness of FS not being able to
modify the Qid(for example the path).
More docs touchup
link to pkg.go.dev, godocs does not display examples correctly.
Adapt srv_test in to an example.
Be a bit more explicit on where to find documentation.
Provide a NopFS for embedding
Rethink what needs to be exported.
Write up some usage docs
Implement session management and slight refactor
NewSrv now takes a function to create a FS object, each 9p session
is given it's own instance of a FS allocated by this function.
This does not mean that all calls in to a FS are synchronous, but
does mean state(like user names from attach) can be stored in the
fs struct itself.
Create was left off the fs interface
first 'working' code
refactor on how responds are handled
current version of 9p is not 9p2020
sketch of Srv looping and refactor