Guard against modification during iteration

[rtheunissen]

The best way I can think of doing this is to keep track of the number of active iterators on the structure, and check if this is > 0 when modifying the structure (during push, remove etc). This will incur a small amount of overhead.

The other question is what to do when that number is > 0?

• throw an exception
• create a warning

[rtheunissen]

See #17