test failure on 32 bit architectures

[davide125]

One test fails when building on i686 or armv7hl

---- src/lib.rs - (line 26) stdout ----
Test executable failed (exit code 101).
stderr:
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `12`,
 right: `8`', src/lib.rs:10:1
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
    src/lib.rs - (line 26)

You can see the full logs at https://koji.fedoraproject.org/koji/taskinfo?taskID=70819940 and https://koji.fedoraproject.org/koji/taskinfo?taskID=70819939 (look at build.log).

[CAD97]

That's this doc test:

beef/src/lib.rs

Lines 26 to 34 in 175938a

	//! ```rust
	//! use std::mem::size_of;
	//!
	//! const WORD: usize = size_of::<usize>();
	//!
	//! assert_eq!(size_of::<std::borrow::Cow<str>>(), 4 * WORD);
	//! assert_eq!(size_of::<beef::Cow<str>>(), 3 * WORD);
	//! assert_eq!(size_of::<beef::lean::Cow<str>>(), 2 * WORD);
	//! ```

The third assert is incorrect on 32 bit targets, since 32 bit targets reexport the 3-usize Cow (#40).

This is nonfatal, but should definitely still be fixed.