func (t *caseFolder) Transform(dst, src []byte, atEOF bool) (nDst, nSrc int, err error) {
	c := context{dst: dst, src: src, atEOF: atEOF}
	for c.next() {
		foldFull(&c)
		c.checkpoint()
	}
	return c.ret()
}
caseFolder implements the Transformer interface for doing case folding.
package cases
import "golang.org/x/text/transform"
type caseFolder struct{ transform.NopResetter }