der_derive: derive(Choice) should implement `DecodeValue`

[dishmaker]

Currently, Choice automatically derives Decode.

But DecodeValue can be derived the same way.

For example, in x509-cert the following DecodeValue for DirectoryString is redundant.

formats/x509-cert/src/ext/pkix/name/dirstr.rs

Lines 64 to 83 in 90b6617

	impl<'a> der::DecodeValue<'a> for DirectoryString {
	type Error = der::Error;
	fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> Result<Self, Self::Error> {
	match header.tag {
	PrintableString::TAG => {
	PrintableString::decode_value(reader, header).map(Self::PrintableString)
	}
	TeletexString::TAG => {
	TeletexString::decode_value(reader, header).map(Self::TeletexString)
	}
	String::TAG => String::decode_value(reader, header).map(Self::Utf8String),
	actual => Err(der::ErrorKind::TagUnexpected {
	expected: None,
	actual,
	}
	.into()),
	}
	}
	}

I think Decode should use DecodeValue in derive(Choice).