Simplify write_to and context manager usage.

[mdboom]

This is an alternative to #116. Fix #107, fix #109.

This is an attempt to fix up context manager usage at the expense of:

• the streaming API which is now a little less convenient (since it relies on having a raw file handle). But that's advanced usage anyway
• It's no longer possible to write_to and then update -- one must open and then update.

@astrofrog: Your thoughts?

[astrofrog]

@mdboom - this looks much more intuitive to me, thanks! I agree that the streaming API is more advanced so it's less important for that to be convenient.

[mdboom]

@nden: This is going to be a bit of a breaking API change. Basically read is now open, and write_to is no longer a context manager (it automatically closes the file and should not be used with with). Let me know if you have any projects that need to be brought over.

[embray]

I guess this is already merged, but I think I like this better too. I've seen other recent cases where people were confused (in a non-ASDF context, I think) about using read/write on objects that are not stream-like objects and don't have the same semantics as, say, file.read and file.write.

[embray]

I feel like it might be desirable to salvage something like the streaming API in the future, only even better--allow writing to a file object, but with protections in place to ensure that it only writes to the correct places, etc. But that can be for another time. In the meantime I also find this more intuitive 👍