Issue Description:

The IndexSet comments/documentation says:

"The range of valid integer values is 0..<INT_MAX-1. Anything outside this range is an error."

(I'm assuming for the rest of this bug report that this is a documentation mistake, and it should actually read "0..<Int.max". See SR-14192 for details.)

The behavior when you exceed this range is inconsistent.

• 0 ... Int.max-1: success

• Int.max (aka NSNotFound): NSRangeException

• Int.min ... -2: NSRangeException

• -1: returns (inserted:true) but then strange things happen

For example, if you make a new IndexSet, and then insert -1, and then insert 0, both insertions claim to succeed and it reports count==2, but min()==nil. (In fact, if -1 is the first value inserted, you can keep re-inserting -1, and count will keep incrementing.)

However, if you insert 0 first and then -1, it reports count==1 and min()==0 as expected, though the -1 insertion returned (inserted:true).

Swift has precondition(), and this would be a great place to use it. It would be better to abort consistently when invalid data is first seen, rather than claiming to succeed and then failing in strange ways later.

I believe only 'insert', 'update', and 'init' would need a range check. All of the other funcs/vars which query the IndexSet already safely ignore out-of-range Ints.