der: add `DecodeValue::decode_value_nested`

[dishmaker]

This PR moves read_nested out of pub(crate) fn read_value.

Such trait function allows us to disable read_nested selectively for some primitive types, like OctetStringRef.

[tarcieri]

@dishmaker have you benchmarked what the actual impact of this is?

[dishmaker]

I did not benchmark it.

[dishmaker]

Need to benchmark this and rebase