Update Flatten() function to handle new dive functionality about validator

[deankarn]

newest benchmarks

$ go test -cpu=4 -bench=. -benchmem=true
PASS
BenchmarkValidateField-4             3000000           434 ns/op         192 B/op          2 allocs/op
BenchmarkValidateStructSimple-4       500000          2760 ns/op         656 B/op         10 allocs/op
BenchmarkTemplateParallelSimple-4     500000          3607 ns/op         656 B/op         10 allocs/op
BenchmarkValidateStructLarge-4        100000         14671 ns/op        4341 B/op         62 allocs/op
BenchmarkTemplateParallelLarge-4      100000         15787 ns/op        4344 B/op         62 allocs/op