Overhaul to_readable_str.

It's a function that prints numbers with underscores inserted for
readability (e.g. "1_234_567"), used by `-Zmeta-stats` and
`-Zinput-stats`. It's the only thing in `rustc_middle::util::common`,
which is a bizarre location for it.

This commit:
- moves it to `rustc_data_structures`, a more logical crate for it;
- puts it in a module `thousands`, like the similar crates.io crate;
- renames it `format_with_underscores`, which is a clearer name;
- rewrites it to be more concise;
- slightly improves the testing.

compiler/rustc_data_structures/src/lib.rs

@@ -76,6 +76,7 @@ pub mod sync;
 pub mod tagged_ptr;
 pub mod temp_dir;
 pub mod thinvec;
+pub mod thousands;
 pub mod transitive_relation;
 pub mod unhash;
 pub mod unord;