ARROW-3741: [R] Add support for arrow::compute::Cast to convert Arrow arrays from one type to anothe

[romainfrancois]

library(arrow)
a <- array(1:10, NA)
a$type()
#> arrow::Int32 
#> int32

b <- a$cast(int16())
b$type()
#> arrow::Int16 
#> int16

^{Created on 2018-11-14 by the reprex package (v0.2.1.9000)}

[romainfrancois]

This needs more work (more tests) and support to cast Table as well, but opening early because I'm seeing some weird stuff:

library(arrow)

a <- array(-(1:3))
a$cast(uint16())
#> Error in Array__cast(self, target_type, options): Invalid: Integer value out of bounds
a$cast(uint16())
#> Error in Array__cast(self, target_type, options): Invalid: Integer value out of bounds
a$cast(uint32())
#> arrow::Array 
#> [
#>   4294967295,
#>   4294967294,
#>   4294967293
#> ]
a$cast(uint32())$type()
#> arrow::UInt32 
#> uint32

a$cast(uint64())
#> arrow::Array 
#> [
#>   -1,
#>   -2,
#>   -3
#> ]
a$cast(uint64())$type()
#> arrow::UInt64 
#> uint64

^{Created on 2018-11-14 by the reprex package (v0.2.1.9000)}

The first two error are what I expect, because safe is true by default in the cast options, but I'd expect the other one to also fail, and not give these results.

[romainfrancois]

closes #2953

[wesm]

In the future when you see something that looks buggy, there's no need to wait to open a JIRA (or go through the mailing list to confirm a bug) =)

[wesm]

Is this ready for review? There's a lot of R PR's open; I would appreciate some tips about which ones need to be reviewed

[romainfrancois]

There's often a rebasing needed between two squashes, probably because some files always change e.g. the generated files or the DESCRIPTION file.

I've added the labels lang-R and ready-for-review I hope it's alright. Perhaps what I can do is mark the one I'd like to be merged next as ready-for-review and the one that really still needs work as WIP

[wesm]

Thanks, "ready-for-review" works for me.

[wesm]

+1. thanks @romainfrancois!