Changes to base

Changes to base:
commit 2d5471cd3ee42406042a030424a1a83308a170fa
Author: Herbert Valerio Riedel <hvr@gnu.org>
Date:   Tue Feb 25 11:04:46 2014 +0100

    Add `withMVarMasked` (re #8818)

    Like `withMVar`, but the @IO@ action in the second argument is executed
    with asynchronous exceptions masked. This completes the `MVar` API,
    which already contained `modifyMVarMasked` and `modifyMVarMasked_`.

    Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>