Contract function and event utilities

[reedsa]

What was wrong?

Utilities made public in #3268 focused on get_event_data and get_function_info.

There are a handful of utility functions that would be helpful for users who do not want to instantiate a Contract instance.

In order to fully make these methods public, refactoring will be necessary to eliminate any circular dependencies.

Private web3._utils.contracts and Contract class methods that may be useful:

• find_matching_fn_abi
• extract_argument_types
• encode_abi
• prepare_transaction
• encode_transaction_data / decode_transaction_data
• get_fallback_function_info
• get_receive_function_info
• create_filter
• build_filter

How can it be fixed?

Make public utilities for contract functions and events. Call the utilities from the Contract class methods.

### Tasks
- [ ] https://github.com/ethereum/web3.py/pull/3355

[pacrob]

Being tracked elsewhere