9.0.0

Various fixes and improvements

• ref: Tokens within a sourcemap are now always sorted by their position in the
  minified file (#91) by @loewenheim.
  Consequently:
  - the type IndexIter and the functions get_index_size, index_iter,
  and idx_from_token have been deleted;
  - the function sourcemap_from_token has been turned into the method
  sourcemap on Token;
  - the idx parameter of SourceMap::get_token now has the type usize.