fixed slice.unique and slice.unique_proc

[kavalee]

Fixed issue #3276 regarding slice.unique and slice.unique_proc returning incorrect slices.

Below is a small test of slice.unique and slice.unique_proc that failed, and are now fixed with this patch.

package demo 

import "core:fmt"
import "core:slice"
main :: proc() {
	s := []int{2,2,2}
	res := slice.unique(s)
	assert(slice.equal(res,[]int{2}))

	res = slice.unique([]int{1,1,1,2,2,3,3,3,3})
	assert(slice.equal(res, []int{1,2,3}))

	equal := proc(a,b: int) -> bool{
		return a == b
	}
	res = slice.unique_proc([]int{1,1,1,2,2,3,3,3,3}, equal)
	assert(slice.equal(res, []int{1,2,3}))
}