move securehash back into the stdlib

changelog.md

@@ -127,8 +127,6 @@ This now needs to be written as:
 - The behavior of ``$`` has been changed for all standard library collections. The
   collection-to-string implementations now perform proper quoting and escaping of
   strings and chars.
-- Removed ``securehash`` stdlib module as it is not secure anymore. The module
-  is still available via ``compiler/securehash``.
 - The ``random`` procs in ``random.nim`` have all been deprecated. Instead use
   the new ``rand`` procs. The module now exports the state of the random
   number generator as type ``Rand`` so multiple threads can easily use their