Fix possible misuse of reflect.SliceHeader vet error

[robbertvanginkel]

A new check was added to vet in go1.16 (golang/go#40701), lint now fails with:

wire/unsafe.go:38:35: possible misuse of reflect.SliceHeader

thriftrw-go/wire/unsafe.go

Lines 28 to 39 in b7d2a80

	// unsafeStringToBytes converts a string into a byte slice without allocating
	// new memory for it with the assumption that the resulting byte slice will not
	// be mutated.
	func unsafeStringToBytes(s string) []byte {
	sh := (*reflect.StringHeader)(unsafe.Pointer(&s))
	sliceHeader := reflect.SliceHeader{
	Data: sh.Data,
	Len: sh.Len,
	Cap: sh.Len,
	}
	return *(*[]byte)(unsafe.Pointer(&sliceHeader))
	}

Currently failing #464. Should fix and rebase.