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/stdx/src/lib.rs

@@ -1,5 +1,4 @@
 //! Missing batteries for standard libraries.
-
 use std::{cell::Cell, fmt, time::Instant};
 
 #[inline(always)]