message: feat(vfs) Make `host_fs` and `mem_fs` mutually exclusives.
  This patch generates a compiler error if `host_fs` and `mem_fs` are
  both enabled at compile-time. It also generates a compiler error if
  none of them have been enabled.
diff: @@ -6,6 +6,12 @@ use std::path::{Path, PathBuf};
  use thiserror::Error;
  use tracing::debug;
  +#[cfg(all(not(feature = "host_fs"), not(feature = "mem_fs")))]
  +compile_error!("At least the `host_fs` or the `mem_fs` feature must be enabled. Please, pick one.");
  +
  +#[cfg(all(feature = "host_fs", feature = "mem_fs"))]
  +compile_error!("The `host_fs` and `mem_fs` features are mutually exclusive.");
  +
  #[cfg(feature = "host_fs")]
  pub mod host_fs;
  //pub mod vfs_fs;
  