Use nested namespaces in the code base

When we transitioned to C++17 there were some namespace hierarchies in the code base that were not updated to use nesting. That is, the code should change from using this:

namespace gkfs {
namespace utils {
...
} // namespace utils
} // namespace gkfs

to this:

namespace gkfs::utils {
...
} // namespace gkfs::utils

The code base should be reviewed and such instances updated.