Compatibility Changes (+ CMake QoL Changes, MSVC Build Fix)

[lenakryger]

Automatically fetch dependencies so that fastft builds "out of the box". Tested with MSVC 2019.

[lenakryger]

I added some more changes to prevent potential issues

1. Prefixes and types for interface types
   • C libraries have no namespaces so it's good practice to prefix everything that you declare.
   • In a complex enough DSP system there could be multiple DSP libraries, all in the global namespace, all with their custom types
     • Imagine if two libraries typedefed "Signal", you'd suddenly have conflicts upon conflicts when including headers from two libraries
     • The same goes for functions, C has no polymorphism
       • Once you declare a symbol, that's it, the name is "taken", any other declaration even if it's with different types is an error
         • Example: "write_to_csv", "write_wav_file" - both of which were in "signal.h"
2. Prefixes for header files and header guards
   • Again, imagine if your lib user uses 3 different libraries in one project and all of them try to include "signal.h"
     • Trying to fix it in the build system (by modifying the include directory order/structure) would be a major pain
   • Two solutions are possible:
     1. Rename "signal.h" to "fastft_signal.h" (and all others similarly)
     2. Make sure that you always include headers with their top directory e.g. "fastft/signal.h", and never add "fastft/" to project include dirs directly
   • The second solution is usually used for larger libraries, so I just renamed all headers by adding a prefix

Additionally, while changing the interface, I also took the liberty of:

• Changing file names to better match the interface (e.g. "pad.h" to "fastft_padding.h", as all the interface functions declared there contain the word "padding")
• Rearranging the word order so that more function names match the pattern fastft_{header/struct-name}_{function-name}
  • e.g. fastft_padding_apply and fastft_padding_init in 'fastft_padding.h' instead of apply_padding and init_padding in 'pad.h'