Ignore specific warnings and make the rest into errors

[frangio]

Fixes #3686

• Ignores code size warnings if we're compiling without optimizations.
• Ignores unreachable code warnings caused by files in contracts/mocks/unreachable.
• Ignores unused parameter warnings that are emitted during coverage analysis (I guess because of coverage transforms).
• Promote the rest of the warnings into errors.
• Makes the revert in ERC721Enumerable._beforeConsecutiveTransfer conditional on the batch size in order to remove the warning about unreachable code that otherwise users will see when they combine with other modules like ERC721Pausable.

[frangio]

Changed the file naming convention as suggested. Still only applies to contracts under contracts/mocks/ because we want to see the warning for the production contracts.