feat(str): from_utf8 adapter

`from_utf8` takes a `Predicate<str>` and turns it into a
`Predicate<u8>`, failing the `eval` if the string is not UTF-8.

I'm considering this to fix #21.  The rest can be handled on an as-need
basis.

Fixes #21

Author: epage

src/boolean.rs

@@ -200,5 +200,6 @@ where
 impl<P, Item> PredicateBooleanExt<Item> for P
 where
     P: Predicate<Item>,
+    Item: ?Sized,
 {
 }

src/str/adapters.rs

@@ -1,3 +1,6 @@
+use std::ffi;
+use std::str;
+
 use Predicate;
 
 /// Predicate adaper that trims the variable being tested.
@@ -20,6 +23,37 @@ where
     }
 }
 
+/// Predicate adaper that converts a `str` predicate to byte predicate.
+///
+/// This is created by `pred.from_utf8()`.
+#[derive(Copy, Clone, Debug)]
+pub struct Utf8Pedicate<P>
+where
+    P: Predicate<str>,
+{
+    p: P,
+}
+
+impl<P> Predicate<ffi::OsStr> for Utf8Pedicate<P>
+where
+    P: Predicate<str>,
+{
+    fn eval(&self, variable: &ffi::OsStr) -> bool {
+        variable.to_str().map(|s| self.p.eval(s)).unwrap_or(false)
+    }
+}
+
+impl<P> Predicate<[u8]> for Utf8Pedicate<P>
+where
+    P: Predicate<str>,
+{
+    fn eval(&self, variable: &[u8]) -> bool {
+        str::from_utf8(variable)
+            .map(|s| self.p.eval(s))
+            .unwrap_or(false)
+    }
+}
+
 /// `Predicate` extension adapting a `str` Predicate.
 pub trait PredicateStrExt
 where
@@ -40,6 +74,24 @@ where
     fn trim(self) -> TrimPedicate<Self> {
         TrimPedicate { p: self }
     }
+
+    /// Returns a `Utf8Pedicate` that adapts `Self` to a `[u8]` `Predicate`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use predicates::prelude::*;
+    /// use std::ffi::OsStr;
+    ///
+    /// let predicate_fn = predicate::str::is_empty().not().from_utf8();
+    /// assert_eq!(true, predicate_fn.eval(OsStr::new("Hello")));
+    /// assert_eq!(false, predicate_fn.eval(OsStr::new("")));
+    /// let variable: &[u8] = b"";
+    /// assert_eq!(false, predicate_fn.eval(variable));
+    /// ```
+    fn from_utf8(self) -> Utf8Pedicate<Self> {
+        Utf8Pedicate { p: self }
+    }
 }
 
 impl<P> PredicateStrExt for P