Revert "Revert "Lowercase shards""

This reverts commit 2ea11e8.

Turns out we need this for Darwin, which sometimes has people using
case-insensitive file systems

src/unit.rs

@@ -67,6 +67,6 @@ pub fn check_unit_dir(unit_dir: PathBuf, _global_index: &GlobalIndex) -> HashSet
 }
 
 pub fn attr_shard_dir(attr: &String) -> OsString {
-    let str : String = attr.chars().take(2).collect();
+    let str : String = attr.to_lowercase().chars().take(2).collect();
     str.into()
 }