Implement r_normalise_encoding()

[DavisVaughan]

Pulled over from https://github.com/r-lib/vctrs/blob/master/src/translate.c

I've left in the maybe-referenced copy semantics, as that seems to work best with vctrs.

Otherwise, I've done minor tweaks to use more of the rlang lib, but nothing major.

I've pulled in all the relevant tests from vctrs, with a few testthat utilities. There are quite a few tests, but I went through each one and none of them feel redundant. I think they are required to ensure we hit all the code paths, but we could move them out of test-c-api.R if you feel it clutters it too much.

[lionel-]

Thanks!

I added it to the C library under the name r_obj_fix_encoding(). Do you like it? If so I'll change the C callable as well.

Merging now so I can depend on this in the unique branch.

[DavisVaughan]

I like the obj_ prefix, but I'd prefer something a bit more specific than fix. Normalise was nice because I could document what a "normalised" CHARSXP was (NA, ASCII, or UTF-8 marked)

Based on the naming scheme we used in arg_match(), I could see:

• r_normalise_encoding() -> obj_normalise_encoding()
• r_attrib_normalise_encoding() -> obj_attrib_normalise_encoding()
• rlang_normalise_encoding() -> rlang_obj_normalise_encoding()
• r_obj_fix_encoding() -> r_obj_normalise_encoding()

[lionel-]

I try to avoid "normalise" and "standardise" because they are long names and have the "s" vs "z" spelling issue. I think "fixing encoding" could still be documented as producing "an object with standard encoding, meaning that ...".

[DavisVaughan]

Alternatively I considered naming this obj_encode_utf8() since that is the intention of the function. Fully ASCII strings are left with an "unknown" encoding (as seen by Encoding()), but they are always valid UTF-8 so that's probably not too confusing

[lionel-]

Right, the "unknown" thing is just an implementation detail of UTF-8 strings in R so that shouldn't have an impact on the naming scheme or documentation. I think I like r_obj_encode_utf8().

[lionel-]

Initial vctrs PR: r-lib/vctrs#565