trie: reduce allocations in hashFullNodeChildren

[omerfirmak]

temporary children array espaces to heap. Directly hash into a fullNode instead.

goos: linux
goarch: amd64
pkg: github.com/ethereum/go-ethereum/trie
cpu: AMD Ryzen 7 5700U with Radeon Graphics         
                      │ /tmp/old.txt │            /tmp/new.txt             │
                      │    sec/op    │   sec/op     vs base                │
HashFixedSize/10-16      22.74µ ± 0%   22.46µ ± 1%   -1.24% (p=0.000 n=10)
HashFixedSize/100-16     67.65µ ± 1%   66.15µ ± 1%   -2.21% (p=0.000 n=10)
HashFixedSize/1K-16      471.4µ ± 4%   517.5µ ± 1%   +9.77% (p=0.000 n=10)
HashFixedSize/10K-16     4.170m ± 1%   3.762m ± 1%   -9.78% (p=0.000 n=10)
HashFixedSize/100K-16    28.05m ± 3%   24.27m ± 2%  -13.48% (p=0.000 n=10)
geomean                  610.5µ        587.9µ        -3.72%

                      │ /tmp/old.txt │             /tmp/new.txt             │
                      │     B/op     │     B/op      vs base                │
HashFixedSize/10-16     7.852Ki ± 0%   5.880Ki ± 0%  -25.12% (p=0.000 n=10)
HashFixedSize/100-16    39.22Ki ± 0%   29.89Ki ± 0%  -23.78% (p=0.000 n=10)
HashFixedSize/1K-16     385.7Ki ± 0%   292.4Ki ± 0%  -24.21% (p=0.000 n=10)
HashFixedSize/10K-16    4.017Mi ± 0%   2.988Mi ± 0%  -25.62% (p=0.000 n=10)
HashFixedSize/100K-16   39.82Mi ± 0%   29.59Mi ± 0%  -25.67% (p=0.000 n=10)
geomean                 456.9Ki        343.2Ki       -24.88%

                      │ /tmp/old.txt │            /tmp/new.txt            │
                      │  allocs/op   │  allocs/op   vs base               │
HashFixedSize/10-16       108.0 ± 0%    101.0 ± 0%  -6.48% (p=0.000 n=10)
HashFixedSize/100-16      570.0 ± 0%    537.0 ± 0%  -5.79% (p=0.000 n=10)
HashFixedSize/1K-16      5.414k ± 0%   5.083k ± 0%  -6.11% (p=0.000 n=10)
HashFixedSize/10K-16     55.56k ± 0%   51.82k ± 0%  -6.73% (p=0.000 n=10)
HashFixedSize/100K-16    554.4k ± 0%   517.2k ± 0%  -6.71% (p=0.000 n=10)
geomean                  6.343k        5.939k       -6.37%

[omerfirmak]

oh, #31902 already exists. Closing.