Add support for pointers to the language.

Pointers can be either uniform or varying, and behave correspondingly.
e.g.: "uniform float * varying" is a varying pointer to uniform float
data in memory, and "float * uniform" is a uniform pointer to varying
data in memory.  Like other types, pointers are varying by default.

Pointer-based expressions, & and *, sizeof, ->, pointer arithmetic,
and the array/pointer duality all bahave as in C.  Array arguments
to functions are converted to pointers, also like C.

There is a built-in NULL for a null pointer value; conversion from
compile-time constant 0 values to NULL still needs to be implemented.

Other changes:
- Syntax for references has been updated to be C++ style; a useful
  warning is now issued if the "reference" keyword is used.
- It is now illegal to pass a varying lvalue as a reference parameter
  to a function; references are essentially uniform pointers.
  This case had previously been handled via special case call by value
  return code.  That path has been removed, now that varying pointers
  are available to handle this use case (and much more).
- Some stdlib routines have been updated to take pointers as
  arguments where appropriate (e.g. prefetch and the atomics).
  A number of others still need attention.
- All of the examples have been updated
- Many new tests

TODO: documentation

llvmutil.h

@@ -52,6 +52,7 @@
 struct LLVMTypes {
     static LLVM_TYPE_CONST llvm::Type *VoidType;
     static LLVM_TYPE_CONST llvm::PointerType *VoidPointerType;
+    static LLVM_TYPE_CONST llvm::Type *PointerIntType;
     static LLVM_TYPE_CONST llvm::Type *BoolType;
 
     static LLVM_TYPE_CONST llvm::Type *Int8Type;
@@ -86,7 +87,7 @@ struct LLVMTypes {
     static LLVM_TYPE_CONST llvm::Type *FloatVectorPointerType;
     static LLVM_TYPE_CONST llvm::Type *DoubleVectorPointerType;
 
-    static LLVM_TYPE_CONST llvm::ArrayType *VoidPointerVectorType;
+    static LLVM_TYPE_CONST llvm::VectorType *VoidPointerVectorType;
 };
 
 /** These variables hold the corresponding LLVM constant values as a
@@ -204,10 +205,4 @@ extern llvm::Constant *LLVMMaskAllOn;
 /** LLVM constant value representing an 'all off' SIMD lane mask */
 extern llvm::Constant *LLVMMaskAllOff;
 
-/** Given an LLVM type, returns the corresponding type for a vector of
-    pointers to that type.  (In practice, an array of pointers, since LLVM
-    prohibits vectors of pointers.
- */
-extern LLVM_TYPE_CONST llvm::ArrayType *LLVMPointerVectorType(LLVM_TYPE_CONST llvm::Type *t);
-
 #endif // ISPC_LLVMUTIL_H