Standardise on using lo for slice functions

We've been sometimes using lo and sometimes using my slices package, and we need to pick one
for consistency. Lo is more extensive and better maintained so we're going with that.

My slices package was a superset of go's own slices package so in some places I've just used
the official one (the methods were just wrappers anyway).

I've also moved the remaining methods into the utils package.

pkg/utils/slice.go

@@ -1,5 +1,7 @@
 package utils
 
+import "golang.org/x/exp/slices"
+
 // NextIndex returns the index of the element that comes after the given number
 func NextIndex(numbers []int, currentNumber int) int {
 	for index, number := range numbers {
@@ -124,3 +126,56 @@ func ValuesAtIndices[T any](slice []T, indices []int) []T {
 	}
 	return result
 }
+
+// returns two slices: the first is for elements that pass the test, the second for those that don't.
+func Partition[T any](slice []T, test func(T) bool) ([]T, []T) {
+	left := make([]T, 0, len(slice))
+	right := make([]T, 0, len(slice))
+
+	for _, value := range slice {
+		if test(value) {
+			left = append(left, value)
+		} else {
+			right = append(right, value)
+		}
+	}
+
+	return left, right
+}
+
+// Prepends items to the beginning of a slice.
+// E.g. Prepend([]int{1,2}, 3, 4) = []int{3,4,1,2}
+// Mutates original slice. Intended usage is to reassign the slice result to the input slice.
+func Prepend[T any](slice []T, values ...T) []T {
+	return append(values, slice...)
+}
+
+// Removes the element at the given index. Intended usage is to reassign the result to the input slice.
+func Remove[T any](slice []T, index int) []T {
+	return slices.Delete(slice, index, index+1)
+}
+
+// Removes the element at the 'fromIndex' and then inserts it at 'toIndex'.
+// Operates on the input slice. Expected use is to reassign the result to the input slice.
+func Move[T any](slice []T, fromIndex int, toIndex int) []T {
+	item := slice[fromIndex]
+	slice = Remove(slice, fromIndex)
+	return slices.Insert(slice, toIndex, item)
+}
+
+// Pops item from the end of the slice and returns it, along with the updated slice
+// Mutates original slice. Intended usage is to reassign the slice result to the input slice.
+func Pop[T any](slice []T) (T, []T) {
+	index := len(slice) - 1
+	value := slice[index]
+	slice = slice[0:index]
+	return value, slice
+}
+
+// Shifts item from the beginning of the slice and returns it, along with the updated slice.
+// Mutates original slice. Intended usage is to reassign the slice result to the input slice.
+func Shift[T any](slice []T) (T, []T) {
+	value := slice[0]
+	slice = slice[1:]
+	return value, slice
+}