patch applied (packages/base): change to use STM, fixing 4 things

[Simon Marlow]

Thu Mar 18 03:44:36 PDT 2010  Simon Marlow <marlowsd at gmail.com>
  * change to use STM, fixing 4 things
    1. there was no async exception protection
    2. there was a space leak (now new value is strict)
    3. using atomicModifyIORef would be slightly quicker, but can
       suffer from adverse scheduling issues (see #3838)
    4. also, the STM version is faster.

    M ./Data/Unique.hs -8 +15