Add Okio-based convenience APIs to thrifty-runtime

[benjamin-bader]

This adds extensions to BufferedSource and BufferedSink, making it easy to convert them into thrifty Transport objects in a fluent style. It also adds fluent builders for making Protocol instances out of Transport instances. This lets us do the following:

val someBigThriftObject = ...

File("/tmp/example.bin")
    .sink()
    .buffer()
    .transport()
    .binaryProtocol()
    .use(someBigThriftObject::write)

In contrast, before this PR one would have to either write a custom transport, or buffer the entire serialized message in memory:

val someBigThriftObject = ...
val buffer = Buffer()
someBigThriftObject.write(BinaryProtocol(BufferTransport(buffer)))
// buffer now holds the entire serialized 

File("/tmp/example.bin")
    .sink()
    .use() { it.write(buffer, buffer.size) }

The new code is about as brief as the old code, is more easily readable (IMO), and has the potential to be much more efficient in that it can partake in okio's segment-sharing and stream reads and writes rather than buffering entire messages into memory at once. In some real-world use cases this is a savings of MB per message of working memory.

It's clearly not hard to write these custom transports, but there's no need for everyone to have to do it themselves.