Get rid of IPositionedSequence<'a>.AllowedToInsertAtArbitraryPlaces

[ForNeVeR]

We have the a TODO in the following code:

praefectus/Praefectus.Core/Diff/Types.fs

Lines 8 to 13 in 9970a26

	type IPositionedSequence<'a> =
	// TODO: Drop this flag, since it is the only mode we use in production code
	abstract AllowedToInsertAtArbitraryPlaces: bool
	abstract MaxCoord: int
	abstract GetItem: coord: int -> 'a option
	abstract AcceptsOn: coord: int * item: 'a -> bool

It should be fixed.