addring.byte_at and string.rune_at

lib/core/src/string.c.ri

@@ -67,6 +67,21 @@ public struct string < Stringable, Hashable, Throwable {
         return unsafe { self.ptr[idx] };
     }
 
+    public func byte_at(self, idx: uint) -> ?uint8 {
+        if idx >= self.len {
+            return none;
+        }
+        return unsafe { self.ptr[idx] };
+    }
+
+    public func rune_at(self, idx: uint) -> ?rune {
+        runes := self.as_runes();
+        if idx >= runes.len {
+            return none;
+        }
+        return runes[idx];
+    }
+
     #[inline]
     public func repeat(self, count: uint) -> Self {
         return if count == 0 {
@@ -505,7 +520,7 @@ public struct string < Stringable, Hashable, Throwable {
         };
     }
 
-    /// Returns an array of all the UTF8 runes in the string `self` which is useful
+    /// Returns an array of all the UTF-8 runes in the string `self` which is useful
     /// if you want random access to them.
     public func as_runes(self) -> []rune {
         mut runes := @vec(rune, self.runes_count());