Provide callers a way to write directly to tmp/

[jmibanez]

Some callers may want to stream bytes into a new mail file directly vs. buffering the entire message blob in memory. To do so, provide a way to return a handle to a file in the maildir tmp/ while allowing cleanup when the handle goes out of scope if it hasn't been renamed into cur/ or new/.

[staktrace]

Based on your stated intent of allowing callers to stream bytes, I made an alternate PR at #48. Would that solve your problem? The idea is that you pass in a Read impl which can provide the bytes on-demand instead of having to buffer them in memory.

[jmibanez]

Unfortunately, that doesn't work for my use case. I'm async, and I have to stream HTTP chunks from an async Stream, so I don't have a single Read object that I can pass in. For me to use #48, I'd have to spin up a separate thread to basically feed between the async source and a Read pipe I hand to e.g. store_new_read(), whereas with the TemporaryMailFile I'm proposing I can simply write_all() each chunk to disk immediately as it arrives.

Additionally, the TemporaryMailFile abstraction does match the whole purpose of having the tmp/ directory in a maildir, and in fact IIRC is why the spec requires it, so that mail arrives "atomically": we can stream bytes to disk first, and then make the mail visible to MUAs by moving it to cur/new when done with a move_tmp_to_{new, cur}() call.

The Read API you propose though is good for callers who already hold a blocking byte source, so it's probably orthogonal to my proposal.

[staktrace]

Ok, sorry for the delay. After looking at this again I think it's a good addition. Merging!