First building blocks for Sharding API: ShardIndex and TransportIdentity #3676

Job	Run time
Basic Checks	6m 47s
Measure coverage	8m 42s
miri	2m 24s
Additional Builds and Concurrency Tests	7m 2s
Release builds and tests	5m 27s
sanitize (address)	4m 17s
sanitize (address, multi-threading)	4m 22s
sanitize (leak)	4m 39s
sanitize (leak, multi-threading)	3m 15s
	46m 55s

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