rstream: Extract some RStream functionality to RBuffer

RBuffer instances represent the internal buffer used by RStreams.

This changes RStream constructor to receive RBuffer pointers and adds a set of
RBuffer methods that expose the lower level buffer manipulation to consumers of
the RStream API.

src/nvim/os/shell.c

@@ -341,7 +341,7 @@ static void system_data_cb(RStream *rstream, void *data, bool eof)
   Job *job = data;
   dyn_buffer_t *buf = job_data(job);
 
-  size_t nread = rstream_available(rstream);
+  size_t nread = rstream_pending(rstream);
 
   dyn_buf_ensure(buf, buf->len + nread + 1);
   rstream_read(rstream, buf->data + buf->len, nread);