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

ispc.cpp

@@ -171,7 +171,7 @@ Target::GetTarget(const char *arch, const char *cpu, const char *isa,
     if (!error) {
         llvm::TargetMachine *targetMachine = t->GetTargetMachine();
         const llvm::TargetData *targetData = targetMachine->getTargetData();
-        t->is32bit = (targetData->getPointerSize() == 4);
+        t->is32Bit = (targetData->getPointerSize() == 4);
     }
 
     return !error;
@@ -284,8 +284,11 @@ llvm::Value *
 Target::SizeOf(LLVM_TYPE_CONST llvm::Type *type) {
     const llvm::TargetData *td = GetTargetMachine()->getTargetData();
     assert(td != NULL);
-    return is32bit ? LLVMInt32(td->getTypeSizeInBits(type) / 8) :
-        LLVMInt64(td->getTypeSizeInBits(type) / 8);
+    uint64_t byteSize = td->getTypeSizeInBits(type) / 8;
+    if (is32Bit || g->opt.force32BitAddressing)
+        return LLVMInt32(byteSize);
+    else
+        return LLVMInt64(byteSize);
 }
 
 
@@ -298,7 +301,12 @@ Target::StructOffset(LLVM_TYPE_CONST llvm::Type *type, int element) {
     assert(structType != NULL);
     const llvm::StructLayout *sl = td->getStructLayout(structType);
     assert(sl != NULL);
-    return LLVMInt32(sl->getElementOffset(element));
+
+    uint64_t offset = sl->getElementOffset(element);
+    if (is32Bit || g->opt.force32BitAddressing)
+        return LLVMInt32(offset);
+    else
+        return LLVMInt64(offset);
 }
 
 
@@ -309,6 +317,7 @@ Opt::Opt() {
     level = 1;
     fastMath = false;
     fastMaskedVload = false;
+    force32BitAddressing = false;
     unrollLoops = true;
     disableAsserts = false;
     disableHandlePseudoMemoryOps = false;