Implement missing equivalent methods from StringMap

This release implements StringMultiMap.Clear() and StringMultiMap.Size, meaning every one of the methods on StringMap have a reasonable equivalent in StringMultiMap (the StringMapSnapshot equivalent being StringMultiMapIterator).

The previous release of 0.1.8 has been removed due to a hard freeze on iterators created from an empty / cleared StringMultiMap.

Iterator removal change

Rewrote StringMultiMapIterator behavior so Remove() takes immediate effect. Attempting to call getters / setters after a Remove() will throw native errors.

This is better behavior, as there is no need to call Next() after removing the current iterator entry (you can now break early).

String getter fixes

Changed string setting (from C++ side) functionality to use SourcePawn's built-in StringToLocal handling.

This should fix memory access violations on Windows 8+ platforms.

First!

Initial implementation.