Add FromStr impl for Yarn

Author: ch3pjw

src/convert.rs

@@ -1,5 +1,7 @@
 use std::borrow::Borrow;
+use std::convert::Infallible;
 use std::fmt;
+use std::str::FromStr;
 use std::str::Utf8Error;
 
 use crate::YarnBox;
@@ -209,6 +211,14 @@ impl From<YarnRef<'_, str>> for String {
   }
 }
 
+impl FromStr for YarnBox<'static, str> {
+  type Err = Infallible;
+
+  fn from_str(s: &str) -> Result<Self, Self::Err> {
+    Ok(Self::copy(s))
+  }
+}
+
 // AsRef / Borrow
 
 impl<Buf> AsRef<Buf> for YarnBox<'_, Buf>