Return more Option types.

[yoshuawuyts]

Some cases to consider:

• calling .prev() on the iterator while at the left nodes
• calling left-child / right-child of a leaf node
• calling left-span / right-span of a leaf node

I'm thinking we should perhaps return the Option type more often in both FlatTree and the Iterator structs.

[yoshuawuyts]

Speak of the devil. Ran into the .prev() issue.

[ralphtheninja]

@yoshuawuyts But we're checking index in .prev()

flat-tree/src/iterator.rs

Lines 65 to 67 in 30f6a07

	if self.offset == 0 {
	return self.index;
	}

[yoshuawuyts]

apparently it can slip through and still crash. Error originates on Iterator, line 69 😞