Add a from_str function that calls out to the associated method on the trait

Author: emberian

src/libstd/from_str.rs

@@ -19,3 +19,7 @@ pub trait FromStr {
     /// string is ill-formatted, the None is returned.
     fn from_str(s: &str) -> Option<Self>;
 }
+
+pub fn from_str<A: FromStr>(s: &str) -> Option<A> {
+    FromStr::from_str(s)
+}

src/libstd/prelude.rs

@@ -41,6 +41,7 @@ pub use result::{Result, Ok, Err};
 // Reexported functions
 pub use io::{print, println};
 pub use iterator::range;
+pub use from_str::from_str;
 
 // Reexported types and traits
 pub use c_str::ToCStr;