SSZ: hashTreeRoot never return

[twoeths]

Describe the bug
This unit test never return

it(`should hash correctly with slice/array data`, () => {
    const accountBalances = {
      balances: []
    };
    for (let i = 0; i < 2; i++) {
      accountBalances.balances.push(new BN("32000000000"));
    }
    const accountBalancesType: AnySSZType = {
      fields: [["balances", { elementType: "uint64", maxLength: 2 }]]
    };
    const hash = hashTreeRoot(accountBalances, accountBalancesType).toString(
      "hex"
    );
    assert(hash);
  });

This is because of the implementation inside hashTreeRoot.
Expected behavior
It should return a hash or throw an error

[twoeths]

I found out that padFor in merkleize is 0.5 which is incorrect and it caused bitLength function to run forever
we expect integer there
Digging into it, I see the following snippet in _hashTreeRoot maybe the root cause

case Type.list:
      value = value as SerializableArray;
      if (isBasicType(type.elementType)) {
        return mixInLength(
          merkleize(
            pack(value, (type as ListType).elementType),
            (type.maxLength * fixedSize(type.elementType)) / BYTES_PER_CHUNK
          ),
          value.length
        );
      } else {

the 2nd parameter in merkleize maybe not correct, it can be like this

Math.floor(type.maxLength * fixedSize(type.elementType) + 31) / BYTES_PER_CHUNK
          )

according to the spec of chunk_count in https://github.com/ethereum/eth2.0-specs/blob/dev/specs/simple-serialize.md

List[B, N] and Vector[B, N], where B is a basic type: (N * size_of(B) + 31) // 32 (dividing by chunk size, rounding up)