Various fixes for compatibility with latest Julia

- contains(str, str) -> !isempty(search()) (maybe there is a better way?)
- has(set, x) -> contains(set, x)
- keys() now returns a KeyIterator, which breaks _warn_extra_opts because it
  wanted a Vector{Symbol}. Because the type declaration shouldn't be necessary
  and KeyIterator isn't exported from Base, I removed it.