READY: Resolve "C++17 support"

Merged Marc Vef requested to merge 134-c-17-support into master

This MR moves GekkoFS to C++17 with the following replacements:

  • CMake modifications
  • Replacing boost::filesystem and boost::optional with std::filesystem and std::optional, respectively
  • Using nested namespaces
  • Using if constexpr

Depends on merge of !66 (merged).

Closes #134 (closed)

Edited by Marc Vef

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