new API for file operations

File operations now always return a channel (or nil on error or
permission denied).

When start_reading() from a filename, you can repeatedly :recv() from
the channel it returns.
When :recv() returns nil, you're at the end of the file. Stop.

When you start_writing() to a filename, you can repeatedly :send() to
the channel it returns.
When you're done writing, :close() the channel. Writes to the file won't
be externally visible until you do.

To make this work I'm now always starting up the scheduler, so I need to
fix sieve.tlv.

Transparently running the scheduler is an abstraction, and whenever I
create an abstraction I always worry about how it might fail. There's
a hopefully-clear error when you read past end of a file.

0 files changed, 0 insertions, 0 deletions