provide as_str() method to return the alphabet characters (#264)

* provide as_string() method to return the alphabet characters

* add docs for as_string()

* put as_string() behind std feature flag

* make as_string() -> as_str() and no_std compatible

* use core::str over std::str

src/alphabet.rs

@@ -1,7 +1,7 @@
 //! Provides [Alphabet] and constants for alphabets commonly used in the wild.
 
 use crate::PAD_BYTE;
-use core::{convert, fmt};
+use core::{convert, fmt, primitive::str};
 #[cfg(any(feature = "std", test))]
 use std::error;
 
@@ -123,6 +123,11 @@ impl Alphabet {
 
         Ok(Self::from_str_unchecked(alphabet))
     }
+
+    /// Create a `&str` from the symbols in the `Alphabet`
+    pub fn as_str(&self) -> &str {
+        core::str::from_utf8(&self.symbols).unwrap()
+    }
 }
 
 impl convert::TryFrom<&str> for Alphabet {
@@ -270,4 +275,11 @@ mod tests {
                 .unwrap()
         );
     }
+
+    #[test]
+    fn str_same_as_input() {
+        let alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+        let a = Alphabet::try_from(alphabet).unwrap();
+        assert_eq!(alphabet, a.as_str())
+    }
 }