Make arrays safer

[igloo]

This is a proposal to make arrays safer (e.g. see #1046).

This is a divergence from Haskell 98.

The patches:

• Add (numElements :: Ix i => a i e -> Int) to IArray class
• Array types get an extra field for numElements, e.g.

     -data UArray i e = UArray !i !i ByteArray#
     +data UArray i e = UArray !i !i !Int ByteArray#
  

  This is a cache of rangeSize(l,u)
• Add safeRangeSize (always returns >= 0)
• Add safeIndex (use unsafeIndex (no Ix inRange check), but check index < numElements)
• unsafeForeignPtrToStorableArray gained an (Ix i) context
• Use the new functions in various places

Suggested deadline: 24 Aug 2007.

[igloo]

Isaac Dupree pointed out a small bug; other than that, no discussion.

[amthrax]

The implementation of safeIndex in this change makes it possible to perform array references that are not inRange. I've detailed the bug and one resolution in ticket #2120.