Issue Description:

Code using the ranged subscript on a MutableCollection instance that doesn't also conform to RangeReplaceableCollection must not change the length of the collection. This is a semantic requirement of MutableCollection that can't be enforced by the compiler, but is caught at runtime in the default implementation and in the standard library's implementing types.

The documentation for MutableCollection.subscript should include a note about this requirement, similar to the one in the MutableCollection protocol's discussion.