der: add `IsConstructed` trait

[dishmaker]

IMPLICIT decoder requires Tagged trait.

https://github.com/RustCrypto/formats/blob/937e611fcbabe929f2ff76320dd33873ea3372dd/der/src/asn1/context_specific.rs#L58-L63

IMPLICIT and EXPLICIT decoders check for constructed bit. But IMPLICIT depends on value.tag() in runtime: https://github.com/RustCrypto/formats/blob/937e611fcbabe929f2ff76320dd33873ea3372dd/der/src/asn1/context_specific.rs#L69-L70

https://github.com/RustCrypto/formats/blob/937e611fcbabe929f2ff76320dd33873ea3372dd/der/src/asn1/context_specific.rs#L161-L164

I think it should check constructed bit statically, like FixedTag.