Repository structure sanity

[sept-en]

Currently, we are having many crates/directories with different Solidity/Rust codebases. This could be misleading as we have multiple connectors in the same repo. In order to make it more clear, we need to move appropriate connectors to some aggregated folders, e.g.:

• erc20-to-nep141 connector which includes all related code for Ethereum-native ERC-20 -> bridged NEAR NEP-141 direction;
• nep141-to-erc20-connector which includes all related code for NEAR-native NEP141 -> bridged Ethereum ERC-20 direction.

All appropriate folders inside aggregated connectors should be renamed accordingly.