Nit: change wording on use of the subtle crate.

It's not that the crate is relying on `subtle` to perform constant-time
arithmetic, because the arithmetic implementations are done in this crate and
it's the fact that they're implemented in a constant-time way that's important.
Using subtle helps with constant-time operations but it is not enough, and
changing the wording of that sentence clarifies that.

(This is a nit).

src/lib.rs

@@ -14,7 +14,7 @@
 //! All operations are constant time unless explicitly noted; these functions will contain
 //! "vartime" in their name and they will be documented as variable time.
 //!
-//! This crate relies on the `subtle` crate for achieving constant time arithmetic.
+//! This crate uses the `subtle` crate to perform constant-time operations.
 
 #![no_std]
 // Catch documentation errors caused by code changes.