Mojibake in error messages with column names on Windows

[krlmlr]

library(dplyr, warn.conflicts = FALSE)
a <- data_frame(a = 1)
names(a) <- "\u5e78"
b <- data_frame(a = 1)
names(b) <- "\u798f"
all.equal(a, b)
#> [1] "Cols in y but not x: �. Cols in x but not y: 幸. "

This will be easier to fix after #2388, look for get_cstring() -- perhaps this method should return string in native encoding.

[krlmlr]

Need to define RCPP_USING_UTF8_ERROR_STRING and consequently use UTF-8 encoding internally.