Optimize decoding CBOR byte string

[fxamacker]

On a new benchmark decoding CBOR byte string to user-defined type with methods, this PR reduces decoding duration by up to 60%. 🚀 On existing benchmarks, this PR reduces decoding duration for CBOR byte string by up to 28% (depending on the data structure and values being decoded).

This PR optimizes decoding of CBOR byte string by:

• moving the BinaryUnmarshaler and TextUnmarshaler interface checks off the hot path
• skipping no-op applyByteStringTextConversion for default decoding options
• caching the byte slice element type check

Benchmarks

Benchmark results for Unmarshal() omit allocs/op and B/op because they are unchanged:

goos: linux
goarch: amd64
pkg: github.com/fxamacker/cbor/v2

NOTE: These benchmark results were produced on a PC running other processes.
Please measure on your own hardware with appropriate settings and workloads.

                                                           │     old      │                 new                 │
                                                           │    sec/op    │   sec/op     vs base                │
Unmarshal/CBOR_byte_string_to_Go_interface_{}                100.05n ± 1%   90.37n ± 1%   -9.68% (p=0.000 n=10)
Unmarshal/CBOR_byte_string_to_Go_[]uint8                      87.85n ± 0%   63.07n ± 1%  -28.21% (p=0.000 n=10)
Unmarshal/CBOR_byte_string_to_Go_cbor.address                158.05n ± 0%   63.38n ± 1%  -59.90% (p=0.000 n=10)
Unmarshal/CBOR_indefinite-length_byte_string_to_Go_[]uint8    309.0n ± 0%   289.1n ± 0%   -6.44% (p=0.000 n=10)
UnmarshalCOSE/128-Bit_Symmetric_Key                           229.2n ± 1%   210.4n ± 0%   -8.20% (p=0.000 n=10)
UnmarshalCOSE/256-Bit_Symmetric_Key                           239.6n ± 0%   218.1n ± 1%   -8.95% (p=0.000 n=10)
UnmarshalCOSE/ECDSA_P256_256-Bit_Key                          416.6n ± 0%   375.5n ± 0%   -9.85% (p=0.000 n=10)
UnmarshalCWTClaims                                            319.8n ± 0%   301.5n ± 0%   -5.72% (p=0.000 n=10)
UnmarshalWebAuthn                                             397.6n ± 0%   377.1n ± 0%   -5.17% (p=0.000 n=10)
UnmarshalCOSEMAC                                              356.2n ± 0%   263.4n ± 1%  -26.05% (p=0.000 n=10)
UnmarshalCOSEMACWithTag                                       407.8n ± 0%   315.4n ± 0%  -22.66% (p=0.000 n=10)

[x448]

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