encoding/gob: make use of reflect.TypeAssert

[apocelipes]

Use "reflect.TypeAssert" to simplify the code.

There are also some performance gains:

goarch: arm64
pkg: encoding/gob
cpu: Apple M4
│ old │ new │
│ sec/op │ sec/op vs base │
EncodeComplex128Slice-10 1.048µ ± 3% 1.048µ ± 1% ~ (p=0.986 n=10)
EncodeFloat64Slice-10 481.5n ± 0% 538.1n ± 0% +11.75% (p=0.000 n=10)
EncodeInt32Slice-10 560.0n ± 1% 562.2n ± 1% ~ (p=0.239 n=10)
EncodeStringSlice-10 713.1n ± 2% 690.1n ± 1% -3.24% (p=0.000 n=10)
EncodeInterfaceSlice-10 16.10µ ± 3% 16.89µ ± 3% +4.94% (p=0.004 n=10)
DecodeComplex128Slice-10 5.507µ ± 2% 5.488µ ± 2% ~ (p=0.617 n=10)
DecodeFloat64Slice-10 3.359µ ± 1% 3.365µ ± 1% ~ (p=0.403 n=10)
DecodeInt32Slice-10 3.296µ ± 1% 3.290µ ± 2% ~ (p=0.926 n=10)
DecodeStringSlice-10 8.397µ ± 2% 8.459µ ± 1% ~ (p=0.796 n=10)
DecodeStringsSlice-10 18.47µ ± 1% 11.14µ ± 1% -39.69% (p=0.000 n=10)
DecodeBytesSlice-10 5.038µ ± 1% 5.039µ ± 1% ~ (p=0.956 n=10)
DecodeInterfaceSlice-10 40.14µ ± 1% 40.60µ ± 1% +1.16% (p=0.001 n=10)
DecodeMap-10 43.43µ ± 1% 44.09µ ± 1% +1.51% (p=0.000 n=10)
geomean 4.451µ 4.335µ -2.62%

                     │      old       │                  new                   │
                     │      B/op      │     B/op      vs base                  │

EncodeComplex128Slice-10 1.0000 ± ? 0.5000 ± ? ~ (p=0.350 n=10)
EncodeFloat64Slice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeInt32Slice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeStringSlice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeInterfaceSlice-10 19.00 ± 5% 20.00 ± 5% +5.26% (p=0.002 n=10)
DecodeComplex128Slice-10 24.55Ki ± 0% 24.53Ki ± 0% -0.10% (p=0.000 n=10)
DecodeFloat64Slice-10 10.56Ki ± 0% 10.54Ki ± 0% -0.22% (p=0.000 n=10)
DecodeInt32Slice-10 9.539Ki ± 0% 9.516Ki ± 0% -0.25% (p=0.000 n=10)
DecodeStringSlice-10 38.18Ki ± 0% 38.16Ki ± 0% -0.06% (p=0.000 n=10)
DecodeStringsSlice-10 63.96Ki ± 0% 40.51Ki ± 0% -36.65% (p=0.000 n=10)
DecodeBytesSlice-10 22.58Ki ± 0% 22.58Ki ± 0% ~ (p=1.000 n=10)
DecodeInterfaceSlice-10 80.47Ki ± 0% 80.47Ki ± 0% ~ (p=1.000 n=10)
DecodeMap-10 48.81Ki ± 0% 48.81Ki ± 0% ~ (p=1.000 n=10) ¹
geomean ² -8.15% ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                     │      old      │                  new                  │
                     │   allocs/op   │  allocs/op   vs base                  │

EncodeComplex128Slice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeFloat64Slice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeInt32Slice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeStringSlice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
EncodeInterfaceSlice-10 0.000 ± 0% 0.000 ± 0% ~ (p=1.000 n=10) ¹
DecodeComplex128Slice-10 149.0 ± 0% 148.0 ± 0% -0.67% (p=0.000 n=10)
DecodeFloat64Slice-10 150.0 ± 0% 149.0 ± 0% -0.67% (p=0.000 n=10)
DecodeInt32Slice-10 149.0 ± 0% 148.0 ± 0% -0.67% (p=0.000 n=10)
DecodeStringSlice-10 1.150k ± 0% 1.149k ± 0% -0.09% (p=0.000 n=10)
DecodeStringsSlice-10 2.158k ± 0% 1.158k ± 0% -46.34% (p=0.000 n=10)
DecodeBytesSlice-10 149.0 ± 0% 149.0 ± 0% ~ (p=1.000 n=10) ¹
DecodeInterfaceSlice-10 3.158k ± 0% 3.158k ± 0% ~ (p=1.000 n=10) ¹
DecodeMap-10 160.0 ± 0% 160.0 ± 0% ~ (p=1.000 n=10) ¹
geomean ² -4.83% ²
¹ all samples are equal
² summaries must be >0 to compute geomean

Updates #62121

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