Migrate all pkg/ files from strings.Contains/HasPrefix/HasSuffix/TrimSpace/
Split/SplitN/Join/ReplaceAll/ToLower/ToUpper/TrimPrefix/TrimSuffix/NewReader/
Builder and fmt.Sprintf/Sprint to their core.* equivalents from dappco.re/go/core.
Retained stdlib where no Core equivalent exists: strings.Map, strings.Trim
(cutset), strings.TrimRight, strings.Index, strings.ContainsAny, strings.Replace
(count), fmt.Sscanf, fmt.Fprintf.
Note: core.Join swaps args vs strings.Join (separator first, then parts...).
Co-Authored-By: Virgil <virgil@lethean.io>