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 (closed) Closes #135 (closed)