arrow

ARROW-3794 [R]: Consider mapping INT8 to integer() not raw()

R vectors of raw types are converted to arrays of type uint8

``` r
arrow::array(as.raw(1:10))$type
#> arrow::UInt8
#> uint8
```

but otoh, arrays of type uint8 are converted to R integer vectors.

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

a <- array((-128):127)$cast(int8())
str(a$as_vector())
#>  int [1:256] -128 -127 -126 -125 -124 -123 -122 -121 -120 -119 ...

a <- array(0:255)$cast(uint8())
str(a$as_vector())
#>  int [1:256] 0 1 2 3 4 5 6 7 8 9 ...
```

<sup>Created on 2019-06-10 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9000)</sup>

Author: Romain Francois <romain@rstudio.com>

Closes #4507 from romainfrancois/ARROW-3794/INT8 and squashes the following commits:

19ff70b1 <Romain Francois> not echo line number in arrowExports.cpp, as this would create some unnecessary changes
524bcb06 <Romain Francois> better tests
68a4aa2b <Romain Francois> raw R vectors are converted to uint8 arrays uint8 arrays are converted to integer vectors