Add EVM builtin function to get ed25519 address mapped to secp256k1 address

[enlight]

We should provide access to account mappings within Solidity contracts so devs can verify signed messages. Currently the address returned by ecrecover in a Solidity contract can't be matched to any DAppChain account (nor msg.sender).

It would look something like this:

// get the address matching the secp256k1 private key that signed the message
address signer = ecrecover(_message, _v, _r, _s);
// get the DAppChain address of the signer
address account = loom_native_api.mappedAccount('eth', signer);

This would need to be implemented as a precompiled EVM contract, we've got an example of doing so in https://github.com/loomnetwork/loomchain/blob/1521b61bc8b4af38a0d9ebc552886f858f006bb3/evm/evm_test.go#L71 and of course ecrecover and friends https://github.com/ethereum/go-ethereum/blob/master/core/vm/contracts.go

[enlight]

Going the other way (DAppChain address -> Eth address) would also be useful:

// lookup the mapping of the DAppChain address of msg.sender to an Eth address
address ethAccount = loom_native_api.mappedAccount('loom', msg.sender, 'eth')
address signer = ecrecover(_message, _v, _r, _s);
// can now compare ethAccount to signer to verify msg.sender signed the message

[Sriep]

Implemented in https://github.com/loomnetwork/loomchain/pull/1526