Implement FromStr for ArrayString

This is very useful for generic code that may want to parse arbitrary
input string into arbitrary other types.

Limitations of the FromStr trait don't allow us to keep the original string slice
inside the CapacityError, unfortunately.

src/array_string.rs

@@ -5,6 +5,7 @@ use std::hash::{Hash, Hasher};
 use std::ptr;
 use std::ops::{Deref, DerefMut};
 use std::str;
+use std::str::FromStr;
 use std::str::Utf8Error;
 use std::slice;
 
@@ -507,6 +508,16 @@ impl<A> Ord for ArrayString<A>
     }
 }
 
+impl<A> FromStr for ArrayString<A>
+    where A: Array<Item=u8> + Copy
+{
+    type Err = CapacityError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::from(s).map_err(CapacityError::simplify)
+    }
+}
+
 #[cfg(feature="serde-1")]
 /// Requires crate feature `"serde-1"`
 impl<A> Serialize for ArrayString<A>