Resolve "Make chunk computations constexpr"

This MR implements the following changes:

- All arithmetic operations based on block sizes are now constexpr.
- Add several new functions, also constexpr: is_aligned(),
is_power_of_2().  - Reimplement log2() to make it faster based on
compiler builtins.  - Rename functions to avoid depending on the concept
of chunk (in preparation for more chunk-agnostic code).  - Add
exhaustive tests for all these functions and enable them in the CI.
- Rename the gkfs::util namespace to gkfs::utils.  - Create the
gkfs::utils::arithmetic namespace for all these functions, as well as
a convenience library/target in CMake for easier testing.

Fixes #137
Closes #135

See merge request !75