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.

lib/vfs/src/lib.rs

@@ -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;