Support 32-bit platforms

[raehik]

We assume native word size to be 64-bit in a few places:

• primitives: Basic.Integers.any(Word|Int), where we increment our parsing address by a constant 8 without checking word size
• some combinators with efficient "bulk" operations (byteString, isolateToNextNull)
• maybe some other places

I've just learned you can #include "MachDeps.h" and then access WORD_SIZE_IN_BITS. We could check this at the various locations where we assume native word size, and provide 32-bit versions as well as existing 64-bit ones.

This would introduce some amount of ugliness for a usecase that I don't think is popular (heck I don't even know how to build a 32-bit GHC). If it still seems reasonable, I'd gladly start on it.

[AndrasKovacs]

I agree It's a good idea to replace literals with MachDeps constants.

[raehik]

Relevant to this, I learned that WORDS_BIGENDIAN is defined in MachDeps.h too, but we never #include it. Fix over at #55 .