Release v0.3.0

New version for Europa

1. Update pallet-contracts to newest version, it contains unstable-interface feature which provides seal_println or new version seal_call or other important feature. The related commit version for substrate is e447c49537e66d0b6e3a408c6ae5c424c7344a7c.
2. Store contract tracing information, state changes data for extrinsics to local and provide some rpc:
   1. europa_extrinsicStateChanges: Return the changed state for an extrinsic (Note: the changed states are not for a whole block, just for an extrinsic).
   2. contractsExt_call: Return the original value for contracts_call with the contract tracing for this call.
   3. contracts_instantiate: Return the origin value for contracts_instantiate with the contract tracing for this contract init.
   4. contractsExt_tracing: Return the contract tracing for an contract call/init extrinsic.
3. Change default TransactionStorageMode from BlockBody to StorageChain.
4. Set contract size to be unlimit. issue: #59.
5. Provide CI to auto pack the linux, windows and macOS pre-compiled executable file. User can download them and run europa directly.

In this version, the database content is not compatible with the data content of the previous version. Thus, you need to delete old storage and restart from new version. (We do not provide the way to migrate from old to new)

And the pre-compiled executable is tested on:

1. macOS is tested on 10.15.7. But notice, this macOS pre-compiled file is used for x86, and we do not provide the M1 version. However we test if you open the Rosetta, the x86 version can run on M1 macOS.
2. windows is tested on 21H1.
3. linux is tested on ubuntu 20.04. We do not provide other linux and ubuntu version binary file, thus, this file even can not run on ubuntu 18.04. If you use linux, we advice you to compile europa by yourself.