[performance] overhaul struct (+ result) caching library for simplicity, performance and multiple-result lookups (#2535)

* rewrite cache library as codeberg.org/gruf/go-structr, implement in gotosocial

* use actual go-structr release version (not just commit hash)

* revert go toolchain changes (damn you go for auto changing this)

* fix go mod woes

* ensure %w is used in calls to errs.Appendf()

* fix error checking

* fix possible panic

* remove unnecessary start/stop functions, move to main Cache{} struct, add note regarding which caches require start/stop

* fix copy-paste artifact... 😇

* fix all comment copy-paste artifacts

* remove dropID() function, now we can just use slices.DeleteFunc()

* use util.Deduplicate() instead of collate(), move collate to util

* move orderByIDs() to util package and "generify"

* add a util.DeleteIf() function, use this to delete entries on failed population

* use slices.DeleteFunc() instead of util.DeleteIf() (i had the logic mixed up in my head somehow lol)

* add note about how collate differs from deduplicate

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

@@ -0,0 +1,76 @@
+package structr
+
+type result[T any] struct {
+	// linked list entry this result is
+	// stored under in Cache.lruList.
+	entry elem[*result[T]]
+
+	// keys tracks the indices
+	// result is stored under.
+	keys []*indexkey[T]
+
+	// cached value.
+	value T
+
+	// cached error.
+	err error
+}
+
+func result_acquire[T any](c *Cache[T]) *result[T] {
+	var res *result[T]
+
+	if len(c.resPool) == 0 {
+		// Allocate new result.
+		res = new(result[T])
+	} else {
+		// Pop result from pool slice.
+		res = c.resPool[len(c.resPool)-1]
+		c.resPool = c.resPool[:len(c.resPool)-1]
+	}
+
+	// Push to front of LRU list.
+	c.lruList.pushFront(&res.entry)
+	res.entry.Value = res
+
+	return res
+}
+
+func result_release[T any](c *Cache[T], res *result[T]) {
+	// Remove from the LRU list.
+	c.lruList.remove(&res.entry)
+	res.entry.Value = nil
+
+	var zero T
+
+	// Reset all result fields.
+	res.keys = res.keys[:0]
+	res.value = zero
+	res.err = nil
+
+	// Release result to memory pool.
+	c.resPool = append(c.resPool, res)
+}
+
+func result_dropIndex[T any](c *Cache[T], res *result[T], index *Index[T]) {
+	for i := 0; i < len(res.keys); i++ {
+
+		if res.keys[i].index != index {
+			// Prof. Obiwan:
+			// this is not the index
+			// we are looking for.
+			continue
+		}
+
+		// Get index key ptr.
+		ikey := res.keys[i]
+
+		// Move all index keys down + reslice.
+		copy(res.keys[i:], res.keys[i+1:])
+		res.keys = res.keys[:len(res.keys)-1]
+
+		// Release ikey to memory pool.
+		indexkey_release(c, ikey)
+
+		return
+	}
+}