This commit implements a substantial re-organisation of the Prelude
It also fixes a couple of small renamer bugs that were reported recently
	(notably, Sven pointed out that we weren't reporting
	unused imports properly)

My original goal was to get rid of all "orphan" modules (i.e. ones
with instance decls that don't belong either to a tycon or a class
defined in the same module).  This should reduce the number of
interface files that have to be read when compiling small Haskell
modules.

But like most expeditions into the Prelude Swamp, it spiraled out
of control.  The result is quite satisfactory, though.

	GONE AWAY:	PrelCCall, PrelNumExtra

	NEW:		PrelReal, PrelFloat, PrelByteArr, PrelNum.hi-boot

(The extra PrelNum.hi-boot is because of a tiresome thin-air Id, addr2Integer,
which used to be in PrelBase.)

Quite a lot of types have moved from one module to another,
which entails some changes to part of the compiler (PrelInfo, PrelMods) etc,
and there are a few places in the RTS includes and even in the driver
that know about these home modules (alas).

So the rough structure is as follows, in (linearised) dependency order
	[this list now appears in PrelBase.lhs]

PrelGHC		Has no implementation.  It defines built-in things, and
		by importing it you bring them into scope.
		The source file is PrelGHC.hi-boot, which is just
		copied to make PrelGHC.hi

		Classes: CCallable, CReturnable

PrelBase	Classes: Eq, Ord, Functor, Monad
		Types:   list, (), Int, Bool, Ordering, Char, String

PrelTup		Types: tuples, plus instances for PrelBase classes

PrelShow	Class: Show, plus instances for PrelBase/PrelTup types

PrelEnum	Class: Enum,  plus instances for PrelBase/PrelTup types

PrelMaybe	Type: Maybe, plus instances for PrelBase classes

PrelNum		Class: Num, plus instances for Int
		Type:  Integer, plus instances for all classes so far (Eq, Ord, Num, Show)

		Integer is needed here because it is mentioned in the signature
		of 'fromInteger' in class Num

PrelReal	Classes: Real, Integral, Fractional, RealFrac
			 plus instances for Int, Integer
		Types:  Ratio, Rational
			plus intances for classes so far

		Rational is needed here because it is mentioned in the signature
		of 'toRational' in class Real

Ix		Classes: Ix, plus instances for Int, Bool, Char, Integer, Ordering, tuples

PrelArr		Types: Array, MutableArray, MutableVar

		Does *not* contain any ByteArray stuff (see PrelByteArr)
		Arrays are used by a function in PrelFloat

PrelFloat	Classes: Floating, RealFloat
		Types:   Float, Double, plus instances of all classes so far

		This module contains everything to do with floating point.
		It is a big module (900 lines)
		With a bit of luck, many modules can be compiled without ever reading PrelFloat.hi

PrelByteArr	Types: ByteArray, MutableByteArray

		We want this one to be after PrelFloat, because it defines arrays
		of unboxed floats.


Other Prelude modules are much easier with fewer complex dependencies.