[chore] bump go structr cache version -> v0.6.0 (#2773)

* update go-structr library -> v0.6.0, add necessary wrapping types + code changes to support these changes

* update readme with go-structr package changes

* improved wrapping of the SliceCache type

* add code comments for the cache wrapper types

* remove test.out 😇

---------

Co-authored-by: tobi <31960611+tsmethurst@users.noreply.github.com>

vendor/codeberg.org/gruf/go-structr/item.go

@@ -0,0 +1,59 @@
+package structr
+
+import (
+	"sync"
+	"unsafe"
+)
+
+type indexed_item struct {
+	// linked list elem this item
+	// is stored in a main list.
+	elem list_elem
+
+	// indexed stores the indices
+	// this item is stored under.
+	indexed []*index_entry
+
+	// cached data with type.
+	data interface{}
+}
+
+var indexed_item_pool sync.Pool
+
+// new_indexed_item returns a new prepared indexed_item.
+func new_indexed_item() *indexed_item {
+	v := indexed_item_pool.Get()
+	if v == nil {
+		v = new(indexed_item)
+	}
+	item := v.(*indexed_item)
+	ptr := unsafe.Pointer(item)
+	item.elem.data = ptr
+	return item
+}
+
+// free_indexed_item releases the indexed_item.
+func free_indexed_item(item *indexed_item) {
+	item.elem.data = nil
+	item.indexed = item.indexed[:0]
+	item.data = nil
+	indexed_item_pool.Put(item)
+}
+
+// drop_index will drop the given index entry from item's indexed.
+// note this also handles freeing the index_entry memory (e.g. to pool)
+func (i *indexed_item) drop_index(entry *index_entry) {
+	for x := 0; x < len(i.indexed); x++ {
+		if i.indexed[x] != entry {
+			// Prof. Obiwan:
+			// this is not the index
+			// we are looking for.
+			continue
+		}
+
+		// Move all index entries down + reslice.
+		copy(i.indexed[x:], i.indexed[x+1:])
+		i.indexed = i.indexed[:len(i.indexed)-1]
+		break
+	}
+}