functional: generalize memoize with normalization parameter.

* lib/std/functional.lua (memoize): Don't rely on tostring
having been monkey-patched to std.string.tostring already, but
also don't automatically load all of std.string into memory
unless memoize is called without a normalization function.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>