stringi usage

[m-muecke]

Currently the only places where stringi is used is in R/helper.R, R/helper_xml.R and R/read_arff.R.
I assume the reason of using stringi is performance, since the following functions have base R equivalents:

• stri_trim_both() : base::trimws()
• stri_startswith_fixed(): base::startsWith()
• stri_endswith_fixed(): base::endsWidth()
• stri_sub():base:sub()

Suggestion would be to either replace the remaining base R string functions with stringi, since startsWith, etc. is still being used or remove stringi as a dependency for consistency.