Add abi::encode_packed

[nikkolasg]

Is your feature request related to a problem? Please describe.
There is currently no solution to have a packed encoding in Rust using ethers, while this is useful for many use cases on Solidity, for example while hashing.

Describe the solution you'd like
an abi::encode_packed()method that takes a list of Token and returns the encoding.

Describe alternatives you've considered
Currently, we have to use another library, which define its own types to achieve it https://github.com/roberts-ivanovs/eth-encode-packed-rs/blob/21962ce04aa62f396ac517fcbea91263dfc373a5/src/lib.rs#L103