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

@@            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 :)