accounts/abi: optimize by not using reflect package

[cuiweixie]

goos: darwin
goarch: arm64
pkg: github.com/ethereum/go-ethereum/accounts/abi
cpu: Apple M1 Pro
                  │   old.txt   │               new.txt               │
                  │   sec/op    │   sec/op     vs base                │
ReadFixedBytes-10   65.87n ± 1%   14.60n ± 0%  -77.84% (p=0.000 n=10)

                  │  old.txt   │              new.txt               │
                  │    B/op    │    B/op     vs base                │
ReadFixedBytes-10   64.00 ± 0%   32.00 ± 0%  -50.00% (p=0.000 n=10)

                  │  old.txt   │              new.txt               │
                  │ allocs/op  │ allocs/op   vs base                │
ReadFixedBytes-10   2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)

[rjl493456442]

The abi unpack is not performance critical. It would prefer to use the reflect.