Speedup VFS::partition

The task of `partition` function is to bin the flat list of paths into
disjoint filesets. Ideally, it should be incremental -- each new file
should be added to a specific fileset.

However, preliminary measurnments show that it is actually fast enough
if we just optimize this to use a binary search instead of a linear
scan.

crates/paths/src/lib.rs

@@ -2,7 +2,7 @@
 //! relative paths.
 use std::{
     convert::{TryFrom, TryInto},
-    ops,
+    io, ops,
     path::{Component, Path, PathBuf},
 };
 
@@ -46,6 +46,9 @@ impl TryFrom<&str> for AbsPathBuf {
 }
 
 impl AbsPathBuf {
+    pub fn canonicalized(path: &Path) -> io::Result<AbsPathBuf> {
+        path.canonicalize().map(|it| AbsPathBuf::try_from(it).unwrap())
+    }
     pub fn as_path(&self) -> &AbsPath {
         AbsPath::new_unchecked(self.0.as_path())
     }