Fix Hardhat compile error when overriding interface functions with public constant variables

[ericglau]

Fixes #1087

When making modified solc input for extracting namespaced storage type information, we previously removed all function bodies and converted all functions' return parameters to bools so that they can be default initialized.

However, if a public constant variable overrides an interface function as in #1087, this causes a mismatch in return type because the interface function's return parameters were modified as above.

Instead, this PR does the following:

• If a function is in a contract and not private, convert to virtual and remove the body
• Otherwise, as long as the function has a body, we know that it is not overriding any other function and no function overrides/implements it. It may be a library function, free function, or private function. In any of these cases, we can convert the return parameters to bools so that they can be default initialized.