PrimaryStorage::clean_dir relies on being able to classify temporary
files from the filename.

I want to introduce more use of nonstd::span<const uint8_t> and I think
that it makes more sense to use unaliased types.

Using the FMT macro explicitly makes invalid format message
constructions compile-time errors instead of run-time errors.

CMake always defines NDEBUG in release builds, so assertions are only
enabled in debug builds. Assertions were however always enabled before
switching to CMake. I prefer keeping assertions enabled in release
builds as well unless they are part of a tight loop.

Fix this by introducing two custom assertion macors:

- ASSERT(condition): Like assert(condition) but always enabled.
- DEBUG_ASSERT(condition): Like assert(condition), i.e. only enabled in
  debug builds.

All usage of assert(condition) is converted to ASSERT(condition) since
none of the assertions are made in performance-critical code.