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.h

@@ -187,7 +187,7 @@ struct Target {
     std::string arch;
 
     /** Is the target architecture 32 or 64 bit */
-    bool is32bit;
+    bool is32Bit;
 
     /** Target CPU. (e.g. "corei7", "corei7-avx", ..) */
     std::string cpu;
@@ -237,6 +237,12 @@ struct Opt {
         it will make sense. */
     bool unrollLoops;
 
+    /** Indicates if addressing math will be done with 32-bit math, even on
+        64-bit systems.  (This is generally noticably more efficient,
+        though at the cost of addressing >2GB).
+     */ 
+    bool force32BitAddressing;
+
     /** Indicates whether assert() statements should be ignored (for
         performance in the generated code). */
     bool disableAsserts;