Fix a bug where some addresses would be unverified due to padding

[rdvorkin]

balanceSlot is padded to '0x' + 64 chars with leading zeros, however contractProof.storageProof[0]?.key isn't. This means that for some addresses (and for some tokens, if more are added) the balance slot will have leading zeros and proof key will not, failing the equality check.

Example:

const balanceSlot = web3.utils.keccak256(defaultAbiCoder.encode(["address", "uint"], ["0xA4b06Da12C746666D08e1163CD18Ab57bA544291", 2]));
const contractProof = await web3.eth.getProof(
        "0x6b175474e89094c44da98b954eedeac495271d0f",
        [balanceSlot],
        17884555);

key=0x9dd74d0da16d86e947a553ada92b0d3f6cee1cfb3c43e4a74617566fbd4c5be
balanceSlot=0x09dd74d0da16d86e947a553ada92b0d3f6cee1cfb3c43e4a74617566fbd4c5be

Or just input 0xA4b06Da12C746666D08e1163CD18Ab57bA544291 at the demo https://lodestar-lightclient.chainsafe.io

[CLAassistant]

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[jeluard]

@rdvorkin Your change makes sense, now I still have an error with your test case even after applying it:

Error: Invalid proof provided: account is not empty (storageHash does not equal KECCAK256_RLP)
    at DefaultStateManager.verifyProof (http://localhost:3000/static/js/bundle.js:18994:15)
    at async fetchAndVerifyAddressBalances (http://localhost:3000/static/js/bundle.js:1052:151)
    at async fetchAndVerifyAccount (http://localhost:3000/static/js/bundle.js:509:31)

Does that ring a bell?