Previously checker objects were created by raw `new` calls, which
necessitated managing and calling their destructors explicitly. This
commit refactors this convoluted logic by introducing `unique_ptr`s that
to manage the ownership of these objects automatically.
This change can be thought of as stand-alone code quality improvement;
but I also have a secondary motivation that I'm planning further changes
in the checker registration/initialization process (to formalize our
tradition of multi-part checker) and this commit "prepares the ground"
for those changes.
ac7c8eb4de