The ornamental design for a slidefastener, as shown and described.
FIG. 1 is a left side view of one element of my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a perspective view thereof;
FIG. 8 is another perspective view thereof;
FIG. 9 is a front elevational view of a plurality of elements of my new design interlocking with each other in a closed position; and,
FIG. 10 is a perspective view of a plurality of elements of my new design interlocking with each other in a partially open position.
The broken line showing of the planar surfaces where the elements are attached in FIG. 10 illustrate environmental matter which form no part of the claimed design.