nostr: add `no_std` support #887

Job	Run time
Format	6s
Build WASM (stable, -p nostr)	1m 28s
Build WASM (stable, -p nostr-sdk)	1m 22s
Build WASM (stable, -p nostr-js)	2m 8s
Build WASM (1.64.0, -p nostr)	3m 44s
Build WASM (1.64.0, -p nostr-sdk)	4m 44s
Build WASM (1.64.0, -p nostr-js)	4m 6s
Build (stable, -p nostr)	2m 17s
Build (stable, -p nostr --no-default-features)	1m 7s
Build (stable, -p nostr --no-default-features --features all-nips)	2m 18s
Build (stable, -p nostr --features blocking)	2m 27s
Build (stable, -p nostr-sdk)	2m 59s
Build (stable, -p nostr-sdk --no-default-features)	2m 19s
Build (stable, -p nostr-sdk --features blocking)	3m 8s
Build (stable, -p nostr-ffi)	2m 21s
Build (stable, -p nostr-sdk-ffi)	3m 39s
Build (1.64.0, -p nostr)	4m 47s
Build (1.64.0, -p nostr --no-default-features)	3m 31s
Build (1.64.0, -p nostr --no-default-features --features all-nips)	4m 24s
Build (1.64.0, -p nostr --features blocking)	5m 57s
Build (1.64.0, -p nostr-sdk)	5m 29s
Build (1.64.0, -p nostr-sdk --no-default-features)	4m 18s
Build (1.64.0, -p nostr-sdk --features blocking)	5m 20s
Build (1.64.0, -p nostr-ffi)	4m 53s
Build (1.64.0, -p nostr-sdk-ffi)	6m 32s
	1h 25m 24s

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