arrow

ARROW-3499: [R] Expose arrow::ipc::Message type

Exposing the `arrow::ipc::Message` and `arrow::ipc::MessageReader` classes with some associated functions:

``` r
library(arrow)

batch <- record_batch(tibble::tibble(x = 1:10))
bytes <- write_record_batch(batch, raw())

reader <- message_reader(bytes)
m <- reader$ReadNextMessage()
m
#> arrow::ipc::Message
m$body()
#> arrow::Buffer
m$type()
#> [1] 1
m <- reader$ReadNextMessage()
m
#> arrow::ipc::Message
m$body()
#> arrow::Buffer
m$type()
#> [1] 3
```

<sup>Created on 2018-11-07 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1.9000)</sup>

Author: Romain Francois <romain@purrple.cat>

Closes #2915 from romainfrancois/ARROW-3499/Message and squashes the following commits:

2561a7ed3 <Romain Francois> ➕ read_schema generic and methods
904f59f93 <Romain Francois> ➕ read_record_batch.{Message,Schema} method
863282632 <Romain Francois> `Message` exposes `body` and `metadata` as actives instead of methods. closer to 🐍 interface
53b5a2575 <Romain Francois> document Message::Type enum
fd6775c82 <Romain Francois> Message and MessageReader
e4fe7917e <Romain Francois> s/ipc___MessageReader__Create/ipc___MessageReader__Open/
b09811d39 <Romain Francois> s/construct/shared_ptr/, add unique_ptr R function. support for unique_ptr.
30a0a1a44 <Romain Francois> expose enum Message::Type
9391a421c <Romain Francois> lint
be9197080 <Romain Francois> R6 class support for arrow::ipc::Message backed by a unique_ptr
033ab9ff7 <Romain Francois> using struct input_parameter<const std::shared_ptr<T>& instead of the heavier Exporter class