fix: replace mainnet TTD so geth mainnet genesis parsing works

[Rjected]

This is a hack to make parsing default geth mainnet genesis.json files work. This all stems from fundamental limitations of serde_json, which does not support numbers larger than u64 by default, meaning the TTD cannot be deserialized. This limitation also makes visit_u128 implementations useless. Theoretically this would be solved by using the arbitrary_precision feature in serde_json, but that has its own problems, which propagates to users:
gakonst/ethers-rs#2206

This replaces the mainnet TTD with the TTD surrounded by quotes, because JsonU256 is able to deserialize strings. This solution does not work for other networks. AFAIK all test networks do not have TTD values larger than u64, so they do not need this hack in the first place.

The hack:

if raw.contains("58750000000000000000000") && !raw.contains("\"58750000000000000000000\"") {
    raw = raw.replace("58750000000000000000000", "\"58750000000000000000000\"");
}

[mattsse]

lgtm,

unfortunately, this is very bad, but it is the best solution here -.-

[mattsse]

since we only call this once during launch, this should be fine and this only affects the mainnet ttd

[mattsse]

ref gakonst/ethers-rs#2617