0.5.1

• The Deserialize implementation for beef::Cow<str> using the impl_serde feature is now specialized, and will produced a borrowed variant if possible, which is particularly useful for serde_json (see #44).
• const_fn feature is internally using a narrower unstable feature gate const_fn_trait_bound (see #41).
• Clarified documentation and doc tests to specify that beef::lean::Cow is two words wide on 64-bit architectures only.