2.1.0: Collectors, iteration, concat varargs, each efficiency

• Heads up: Removed sum() operation, it can easily be rewritten to
  either $sum= $seq->reduce(0, function($a, $b) { return $a + $b; })
  or $sum= $seq->collect(Collectors::summing()) which are more flexible.
  (@thekid)
• Made functions to Collectors::summing and Collectors::averaging
  optional. If omitted, the elements themselves will be used for the
  numbers.
  (@thekid)
• Added support for referring to instance methods via string. Depends
  on xp-framework/core#44, see pull request #17 - @thekid
• Added Sequence::toMap() - see pull request #10 - @thekid
• Added Sequence::iterator() - see pull request #16 - @thekid
• Added the ability to pass a variable number of arguments to
  Sequence::concat(). At the same time, the method is changed to
  be more liberal in what it accepts. See pull request #13.
  (@thekid)
• Implemented variant of Sequence::each() which iterates elements
  without callback and is thus more memory-efficient than calling
  e.g. toArray() and discarding the results. See pull request #12.
  (@thekid)