Add alignment-restricted ForeignPtr allocation actions

[nicolast]

Whilst GHC.ForeignPtr? exports some actions to allocate buffers containing an element of some type, or of some specific byte-length efficiently, and there's an implementation of newAlignedPinnedByteArray# used internally, there's no way to request such buffer of a given size and at a given alignment.

The ability to enforce a specific alignment for some memory buffer is important e.g. when working with SIMD instructions (which is my original use-case). My attempts to work-around the lack of this function failed miserably (with a reproducible test-case) as mentioned in [1].

[1]  http://www.haskell.org/pipermail/glasgow-haskell-users/2012-July/022579.html

[nicolast]

Add alignment-restricted allocation actions to GHC.ForeignPtr?

[nicolast]

Using attachment:ticket:7067:0001-Add-alignment-restricted-ForeignPtr-allocation-actio.patch and an adapted mallocVector implementation (see mallocVectorMine3 at  https://gist.github.com/3084806#LC129) the corruption as found earlier (using mallocVectorMine1 or mallocVectorMine2) no longer occur.

[simonmar]

Patch looks good to me.

[simonmar]

Applied, thanks!