Safer handling of 'illegal' characters in TSV input.

[DesiQuintans]

A few times now, I've gotten hard-to-debug errors caused by extended characters in column renames and variable descriptions. The most recent one was caused by • in a variable label that caused describe_with_dictionary() to fail.

TSV input should always be converted. Potential column renames should probably be passed through iconv(c(...), to = "ASCII//TRANSLIT"). Potential variable labels and factor levels should probably be passed through iconv(c(...), to = "ASCII", sub = "c99"). The latter converts extended characters into "\\u00b0" codes.