Support for adding to an associative array's domain via the array itself

This code allows for one to implicitly add to an associative array's domain via that array. This makes associative arrays a little more pleasant to work with.

Before:

myDom += newKey;
myArr[newKey] = foo;

After:

myArr[newKey] = foo; // newKey added to backing domain if not present.

An important restriction is that the array and domain must share a 1:1 relationship. More simply, the domain must be backing only one array. The motivation for this restriction is that it may be surprising that other arrays' index sets are modified when it appears that we are only modifying one array. If this restriction is violated, the program will halt.

An 'out of bounds' halt will still occur if attempting to read from a bad index:

Before and after:

writeln(myArr[badKey]); // if 'badKey' is not in myArr's index set, the usual error will be encountered.
The problem is that in either case, _findFilledSlot() is looking at .status and .idx and those could potentially be changing due to a call to _add().  It's possible for _findFilledSlot() to see .status as full, and then see an old value for .idx.  This isn't a problem if the old value doesn't match the requested value, but if it does, then _findFilledSlot() will incorrectly return true.  False negatives are okay here, but false positive will result in incorrect uses of the slot.