Support UTF-16 encodings

[jimhester]

# This is the Byte Order Mark:
bom <- as.raw(c(255, 254))
# This is the text.
text <- "1\t2\n"

# Converted to UTF-16LE
text_utf16 <- iconv(text,from="UTF-8",to="UTF-16LE", toRaw = TRUE)[[1]]

# Write the BOM and the text to a file
tmp_file_name <- tempfile()
fd <- file(tmp_file_name, "wb")
writeBin(bom, fd)
writeBin(text_utf16, fd)
close(fd)

# read the file and use iconv to convert it to UTF-8
con <- file(tmp_file_name, "rb")
x <- readBin(con, "raw", n = 100)
vroom::vroom(iconv(list(x), from = "UTF-16LE", to = "UTF-8", toRaw = TRUE), col_names = FALSE)
#> Observations: 1
#> Variables: 2
#> dbl [2]: X1, X2
#> 
#> Call `spec()` for a copy-pastable column specification
#> Specify the column types with `col_types` to quiet this message
#> # A tibble: 1 x 2
#>      X1    X2
#>   <dbl> <dbl>
#> 1     1     2

^{Created on 2019-06-20 by the reprex package (v0.3.0)}

So we should basically be able to do this iconv conversion on the fly if given a multi-byte encoding.