Renamed methods for consistency and accuracy (#46)

* `Collection::filtered()` renamed to `filter()`
* `AssociativeInterface::combine()` renamed to `copyAndMerge()`
* `IterableInterface::filtered()` renamed to `copyAndFilter()`
* `IterableInterface::map()` renamed to `copyAndMap()`
* `MutableIterableInterface::apply()` renamed to `map()`
* `SequenceInterface::sorted()` renamed to `copyAndSort()`
* `SequenceInterface::reversed()` renamed to `copyAndReverse()`
* `SequenceInterface::join()` renamed to `copyAndAppend()`
* `Set::isStrictSuperset()` renamed to `isProperSuperset()`
* `Set::isStrictSub()` renamed to `isProperSubset()`
* `Set::complement()` renamed to `diff()`
* `Set::complementInPlace()` renamed to `diffInPlace()`
* Added `Set::isIntersecting()`
* Added `Set::symmetricDiff()`
* Added `Set::symmetricDiffInPlace()`