Performance & functionality improvements

AbstractFixedCollectionPlus:
- The methods "exists", "map", and "filter" now accept strings or arrays for their "$func" parameter.  "is_callable" is used to determine if the value is valid.

AbstractCollectionPlus:
- Completely doing away with previous internal position mechanism
- All position-manipulation methods now directly interact with the private class var "_storage"'s internal pointer.
- Updating "exists", "map", and "filter" methods to same implementation as AbstractFixedCollectionPlus
Some comment changes.
Fixing callable bug

- More intelligent closure usage.
Updating README.md