Add StiOfXXX in xslice

[Aoi-hosizora]

Add StiOfXXX functions in xslice just like ItsOfXXX.

ahlib/xslice/xslice_helper.go

Lines 59 to 65 in 21424a4

	func ItsOfString(slice []interface{}) []string {
	out := make([]string, len(slice))
	for idx := range slice {
	out[idx] = slice[idx].(string)
	}
	return out
	}

func StiOfString(slice []string) []interface{} { }

[Aoi-hosizora]

Done in 9a81510.