Various fallback improvements

[seanyoung]

Allow the following:

contract A {
    // Take raw calldata as argument and return return data
    fallback(bytes calldata input) external payable (bytes memory) {}
}

Allow a function to be called fallback:

contract A {
    // Will give a warning that's not a fallback function,
    // just a regular functon
    function fallback() public {}
}

This should make proxy contracts much easier to implement.

Also fixes all fallback/receive related failures in the evm tests.

[codecov[bot]]

Codecov Report

Attention: 35 lines in your changes are missing coverage. Please review.

Comparison is base (00bcabb) 88.44% compared to head (1a4cef6) 88.42%.

Files	Patch %	Lines
src/lir/converter/expression.rs	0.00%	23 Missing :warning:
src/codegen/cfg.rs	0.00%	10 Missing :warning:
src/codegen/mod.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1611 +/- ## ========================================== - Coverage 88.44% 88.42% -0.03% ========================================== Files 150 150 Lines 68322 68503 +181 ========================================== + Hits 60429 60572 +143 - Misses 7893 7931 +38 ```

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[xermicus]

@seanyoung I'll be preparing a release in the upcoming days, I think this would be nice to have in :)