arrow

ARROW-3732 [R] Add functions to write RecordBatch or Schema to Message value, then read back

``` r
library(arrow, warn.conflicts = FALSE)

s <- schema(x = int32())
bytes <- s$serialize()
msg <- read_message(bytes)

s2 <- read_schema(msg)
s == s2
#> [1] TRUE

b <- record_batch(x = 1:10)
bytes <- b$serialize()
msg <- read_message(bytes)
b2 <- read_record_batch(msg, s)
b == b2
#> [1] TRUE
```

Author: Romain Francois <romain@rstudio.com>

Closes #4673 from romainfrancois/ARROW-3732/Schema_serialize and squashes the following commits:

395f6b5f8 <Romain Francois> + Schema$Equals()
4d3fe841e <Romain Francois> + read_schema(<Message>)
899cd4720 <Romain Francois> read_message() handles raw vectors (and anything that BufferReader() can handle
b0b910e75 <Romain Francois> + Schema$serialize() and additional tests in read_message()
666bd0efc <Romain Francois> move implementation of Schema__*() in schema.cpp