Safe optimization for COSE (RFC 8152) and WebAuthn

[x448]

It would be nice to be able to decode CBOR maps with integer keys to Go struct, and vice versa.

This feature would speed up and simplify using this library for COSE. It would also speed up WebAuthn since that uses COSE.

[fxamacker]

Thanks. I'll look into it for v1.3.0.

[fxamacker]

Decoding COSE to struct is 42.125% faster than decoding to map[int]interface{} and 42.105% less B/op.

Benchmarks of decoding COSE to map[int]interface{} vs. struct:

BenchmarkDecodeCOSE/COSE_to_Go_map[int]interface_{}-2                         	 1000000	      1600 ns/op	     456 B/op	      13 allocs/op
BenchmarkDecodeCOSE/COSE_to_Go_cbor_test.coseKey-2                            	 2000000	       926 ns/op	     264 B/op	      13 allocs/op