add a precondition to prevent slice assignment out-of-bounds

[glessard]

This adds a precondition that will trap if the slice on the right hand side of an assignment contains any index that is absent in the slice on the left hand side of the assignment.

[glessard]

This is in keeping with usual standard library behaviour, but may still be worth discussing. The current behaviour (in 1.0.1) is to ignore indices that are not present in the slice in the left hand side of the assignment.

Do we want to trap in this case? If we want this, the tests need an update.

Note that we could also require the index sets to be equal on both sides of the assignment.

[jmschonfeld]

In the most recent review of RangeSet, this extension and conformance to MutableCollection have both been removed and the removals are reflected in the implementation at swiftlang/swift#69766

[glessard]

Thanks @jmschonfeld!