Fix spurious allocation in hasher.Merkleize

[karalabe]

This PR fixed a leaky allocation that happens in merkleizeImpl.

Specifically, https://github.com/ferranbt/fastssz/blob/main/hasher.go#L379 expands the input slice:

		if oddNodeLength {
			// is odd length
			input = append(input, zeroHashes[i][:]...)
			layerLen++
		}

but the problem is that the input slice is a sub-slice of h.buf, so if the expansion needs to allocate a new buffer because we're at capacity, then the newly allocated memory area gets thrown away and will not be piled into h.buf to be used subsequently. The result is that calls in a loop will allocate in a loop.

You can verify this by hashing DepositData for example, where the last field is 96 bytes, requiring a 32 byte expansion, but since it's at the end of the buffer, it will trigger the case.

[karalabe]

Here's a repro for the bug and the fix.

func BenchmarkHashTreeRootAllocBug(b *testing.B) {
	obj := new(DepositData)
	readValidGenericSSZ(nil, "../eth2.0-spec-tests/tests/mainnet/deneb/ssz_static/DepositData/ssz_random/case_4", obj)

	b.ReportAllocs()
	b.ResetTimer()

	hh := ssz.NewHasherWithHashFn(gohashtree.HashByteSlice)
	for i := 0; i < b.N; i++ {
		obj.HashTreeRootWith(hh)
		hh.Reset()
	}
}

Without the PR:

go test ./spectests --run=NONE --bench=BenchmarkHashTreeRootAllocBug

BenchmarkHashTreeRootAllocBug-12    	 3209919	       358.4 ns/op	     192 B/op	       1 allocs/op

With the PR:

go test ./spectests --run=NONE --bench=BenchmarkHashTreeRootAllocBug

BenchmarkHashTreeRootAllocBug-12    	 3425565	       335.1 ns/op	       0 B/op	       0 allocs/op

[Pjrich1313]

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