feat(ics20): introduce emulated ICS20 via custom CW20-ICS20 contract (#147)

fixes #136 

In this PR we introduce the CW20-ICS20 contract forked from
https://github.com/CosmWasm/cw-plus/blob/main/contracts/cw20-ics20/README.md#cw20-ics20.

Initial modification made:
- change from JSON to eth abi encoding for the packet

This is a breaking change for EVM and the according solidity ICS20
contract must be updated to reflect this.