der: add `IsConstructed` trait, impl'ed on any `FixedTag` #2379

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	56s
build (1.81.0, wasm32-unknown-unknown)	1m 1s
build (stable, wasm32-unknown-unknown)	51s
test (1.81.0)	2m 36s
build (stable, thumbv7em-none-eabi)	56s
fuzz	2m 10s
test (stable)	2m 22s
minimal-versions	-1s
	10m 51s

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