accounts: use hash pool in TextHash

[cuiweixie]

func BenchmarkTextHash(b *testing.B) {
	// warm up
	_ = TextHash([]byte("Hello Joe"))

	for b.Loop() {
		_ = TextHash([]byte("Hello Joe"))
	}
}

goos: darwin
goarch: arm64
pkg: github.com/ethereum/go-ethereum/accounts
cpu: Apple M1 Pro
            │   old.txt   │              new.txt               │
            │   sec/op    │   sec/op     vs base               │
TextHash-10   505.1n ± 1%   484.2n ± 0%  -4.15% (p=0.000 n=10)

            │  old.txt   │              new.txt               │
            │    B/op    │    B/op     vs base                │
TextHash-10   168.0 ± 0%   120.0 ± 0%  -28.57% (p=0.000 n=10)

            │  old.txt   │              new.txt               │
            │ allocs/op  │ allocs/op   vs base                │
TextHash-10   5.000 ± 0%   4.000 ± 0%  -20.00% (p=0.000 n=10)

[rjl493456442]

This is not performance critical. I prefer to not use the unsafe here.