I'm very grateful to @jacobwilliams for starting out this modernization of the fftpack library. However, with the developments that have happened with stdlib over the past couple of years, I feel like we should go one step further. This PR is the first of an upcoming series aiming at bringing fftpack up to the same quality and performance level as stdlib. It contains the following changes:

• CMake build now closely parallels that of stdlib (with fypp being turned off for the moment).
• The meson build system has been removed for now for the sake of simplicity but can be added back eventually if users ask for it.
• The set of Github Actions now mimicks closely that of stdlib, including:
  • CI.yml : combination of unix-based OS and gfortran or intel compilers
  • CI_windows.yml : uses msys2-build for Windows
  • fpm-deployment : deploys a dedicated fpm branch (if we follow stdlib with the main branch being dedicated to the raw fypp files and CMake build) along with code coverage (via codecov)
  • PR-review.yml : simple action looking for misspelling.

The next few PRs will focus on further modernizing the existing code base (usage of do concurrent wherever appropriate, modules and submodules, etc). Note that I already started modernizing the zfft and dfft and get up a 1.3x speed-up simply by using some do concurrent facilitating the compiler's optimizations of certain loops.

As I ain't sure who is actively following this repo, I'll ping the linalg crew for now @perazz, @jvdp1 and @jalvesz.