Add __toString() to the BigNumber interface

src/BigDecimal.php

@@ -805,9 +805,7 @@ class BigDecimal implements BigNumber, \Serializable
     }
 
     /**
-     * Returns a string representation of this number.
-     *
-     * @return string
+     * {@inheritdoc}
      */
     public function __toString()
     {

src/BigInteger.php

@@ -638,7 +638,7 @@ class BigInteger implements BigNumber, \Serializable
     }
 
     /**
-     * @return string
+     * {@inheritdoc}
      */
     public function __toString()
     {

src/BigNumber.php

@@ -8,6 +8,8 @@ namespace Brick\Math;
 interface BigNumber
 {
     /**
+     * Converts this number to a BigInteger.
+     *
      * @return BigInteger
      *
      * @throws ArithmeticException If this number cannot be safely converted to a BigInteger.
@@ -15,6 +17,8 @@ interface BigNumber
     public function toBigInteger();
 
     /**
+     * Converts this number to a BigDecimal.
+     *
      * @return BigDecimal
      *
      * @throws ArithmeticException If this number cannot be safely converted to a BigDecimal.
@@ -22,7 +26,16 @@ interface BigNumber
     public function toBigDecimal();
 
     /**
+     * Converts this number to a BigRational.
+     *
      * @return BigRational
      */
     public function toBigRational();
+
+    /**
+     * Returns a string representation of this number.
+     *
+     * @return string
+     */
+    public function __toString();
 }

src/BigRational.php

@@ -457,7 +457,7 @@ class BigRational implements BigNumber, \Serializable
     }
 
     /**
-     * @return string
+     * {@inheritdoc}
      */
     public function __toString()
     {