Improve parsing of Solidity 0.8

[montyly]

These are two hotfixes to better support Solidity 0.8.

I am expecting more changes to be required, and we need to investigate more AST modifications with 0.8, but I wanted both fixes to be included in today's release

Fix (partially)

• Are these false positives? variable should be constant and contract does not implement functions #794
• Upgrade from solidity 0.7 to 0.8 caues a lot of unimplemented-functions errors #814

Related:

• Improve 0.8 support #795