superchain: export single mapping instead of three separate ones

[bitwiseguy]

Context: #386 (comment)

This would introduce a breaking change for op-node, op-geth and any other downstream software that imports the superchain package (e.g. op-reth), but it would be a nice cleanup. Right now the superchain package populates three mappings:

var OPChains = map[uint64]*ChainConfig{}
var Addresses = map[uint64]*AddressList{}
var GenesisSystemConfigs = map[uint64]*SystemConfig{}

After moving to toml config files, all of the data is contained in the OPChains mapping so we could remove the Addresses and GenesisSystemConfig ones to reduce duplicated data. But the downstream software would need to be updated to parse the updated ChainConfig struct for that data