Proper connections

[richfitz]

In earlier versions of the README (3a20372 and earlier) I wrote:

## Why not a connection object?

A proper connection could be nice but there are at least three issues stopping this:

1. `sodium` does not support streaming encryption/decryption.  It might be possible (bindings to node and swift have it).  In general this would be great and allow the sort of cool things you can do with streaming large data in curl.
2. R plays pretty loose and free with creating connections when given a filename; `readRDS`/`saveRDS` will open files with decompression on in binary mode, `read.csv`/`write.csv` don't.  `write.table` adds encoding information when openning the connection object.  The logic around what happens is entirely within the functions themselves so is hard to capture in a general way.
3. Connection objects look like a pain to write.

There are still problems with the approach I've taken:

* Appending does not work: we'd need to unencrypt the file first for that to be OK
* Non-file arguments are going to suck (though it's possible that something could be done to detect connections)

In the end, you can always write things out however you like and use `encrypt_file` to encrypt the file afterwards.

This issue is just to record that proper connections might be nice to have

[jeroen]

You should know that BDR has banned custom connections. Packages using the connections API now raise a NOTE and get rejected from CRAN.

[richfitz]

Ah yes, I remember some complaints about that - it was a retrospective API change or something?