Add Solidity 0.7 support

[montyly]

https://solidity.ethereum.org/2020/07/28/solidity-v0.7.0-release-announcement/

• [ ] Inline Assembly: Disallow . in user-defined function and variable names.
• [ ] Inline Assembly: Slot and offset of storage pointer variable x are accessed via x.slot and x.offset instead of x_slot and x_offset.
• [ ] JSON AST: Mark hex string literals with kind: "hexString".
• [ ] JSON AST: Remove members with null value from JSON output.
• [ ] Reference Resolver: using A for B only affects the contract it is mentioned in and not all derived contracts
• [ ] Parser: Introduce Unicode string literals: unicode"😃".

[montyly]

I forgot to close this issue with https://github.com/crytic/slither/releases/tag/0.6.14.

If I am not mistaken, only slot was missing, which is covered by https://github.com/crytic/slither/pull/833

If we find missing features, we will create individual gh issue.