Implement event selectors

[seanyoung]

contract e {
    event E();
    function f() public {
       bytes32 selector = E.selector;
    }
}

[codecov[bot]]

Codecov Report

Merging #1584 (a6d06bf) into main (855b22b) will increase coverage by 0.01%. The diff coverage is 88.05%.

@@            Coverage Diff             @@
##             main    #1584      +/-   ##
==========================================
+ Coverage   87.55%   87.56%   +0.01%     
==========================================
  Files         133      133              
  Lines       64118    64212      +94     
==========================================
+ Hits        56137    56228      +91     
- Misses       7981     7984       +3     

Files	Coverage Δ
src/abi/anchor.rs	90.53% <100.00%> (+0.14%)	:arrow_up:
src/abi/ethereum.rs	92.04% <100.00%> (ø)
src/abi/polkadot.rs	97.75% <100.00%> (ø)
src/bin/idl/mod.rs	77.88% <100.00%> (ø)
src/bin/languageserver/mod.rs	79.04% <100.00%> (ø)
src/codegen/events/mod.rs	100.00% <ø> (ø)
src/codegen/events/polkadot.rs	100.00% <100.00%> (ø)
src/codegen/events/solana.rs	100.00% <100.00%> (ø)
src/codegen/expression.rs	95.25% <100.00%> (+0.01%)	:arrow_up:
src/sema/expression/member_access.rs	94.24% <100.00%> (+0.51%)	:arrow_up:
... and 7 more

... and 1 file with indirect coverage changes