Implement fallback functions

[DemiMarie]

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I am porting the POA Networks Bridge Contracts from Solidity to Rust. However, the Solidity code makes use of fallback functions in several places, perhaps most notably here. The eth_abi macro doesn’t support fallback functions.

[NikVolf]

Hi, pwasm-abi does not support fallback, but nothing prevents you from handling input data bytes manually

You just need to use input function to fetch those bytes and handle them similar to how proxy contract does it

[NikVolf]

But we would like to support fallback functions anyway for different use cases