Server function streaming with serializable types

[ealmloff]

This PR handles serialization and deserialization for custom types with streaming server functions (#1284)

It adds a new StreamingJson encoding which serializes each item in the stream with serde_json. The implementation is a fairly straight forward combination of the streaming bytes and json codecs with two exceptions:

1. The content type is the default streaming type instead of JSON because if the chunks are read as merged chunks they cannot be parsed as valid json
2. When using a custom stream type with StreamingJson, this PR requires the bound Stream<Item=T> in order to make the T type used in the trait implementation. We don't use the bound, but it makes the T type constrained. Alternatively, I could add an unused generic to StreamingJson or FromReq, but I would imagine both of those would effect user code more

[gbj]

This looks good to me. Thanks very much for the addition!

[boblehest]

I don't think this is correct; it decodes each chunk read from the stream using serde_json::from_slice, but doesn't seem to do any framing to keep track of message boundaries. E.g. sending 1 immediately followed by 2 will usually be read as the integer 12, and sending 1.0 immediately followed by 2.0 will usually fail when it tries to decode it as 1.02.0.

[ealmloff]

I don't think this is correct; it decodes each chunk read from the stream using serde_json::from_slice, but doesn't seem to do any framing to keep track of message boundaries. E.g. sending 1 immediately followed by 2 will usually be read as the integer 12, and sending 1.0 immediately followed by 2.0 will usually fail when it tries to decode it as 1.02.0.

StreamingJson uses the http chunked encoding. HTTP streaming is chunked with a built in delimiter instead of a single uninterrupted stream of binary data. More info here

[boblehest]

I don't think this is correct; it decodes each chunk read from the stream using serde_json::from_slice, but doesn't seem to do any framing to keep track of message boundaries. E.g. sending 1 immediately followed by 2 will usually be read as the integer 12, and sending 1.0 immediately followed by 2.0 will usually fail when it tries to decode it as 1.02.0.

StreamingJson uses the http chunked encoding. HTTP streaming is chunked with a built in delimiter instead of a single uninterrupted stream of binary data. More info here

Thank you for the explanation. Which part of the code is responsible for adding the Transfer-Encoding header? I tried using StreamingJson in my app but it fails in the way I described in my previous comment -- I might get around to providing a test case later this week.