Split constant_time::Word into a submodule.

```
git difftool HEAD^1:src/constant_time.rs src/constant_time/word.rs
```

src/constant_time.rs

@@ -17,14 +17,9 @@
 use crate::{c, error};
 
 mod boolmask;
+mod word;
 
-#[cfg(target_pointer_width = "64")]
-pub(crate) type Word = u64;
-
-#[cfg(target_pointer_width = "32")]
-pub(crate) type Word = u32;
-
-pub(crate) use self::boolmask::BoolMask;
+pub(crate) use self::{boolmask::BoolMask, word::Word};
 
 /// Returns `Ok(())` if `a == b` and `Err(error::Unspecified)` otherwise.
 /// The comparison of `a` and `b` is done in constant time with respect to the

src/constant_time/word.rs

@@ -0,0 +1,19 @@
+// Copyright 2015-2016 Brian Smith.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+#[cfg(target_pointer_width = "64")]
+pub(crate) type Word = u64;
+
+#[cfg(target_pointer_width = "32")]
+pub(crate) type Word = u32;