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

lex.ll

@@ -112,9 +112,12 @@ int64 { return TOKEN_INT64; }
 launch { return TOKEN_LAUNCH; }
 NULL { return TOKEN_NULL; }
 print { return TOKEN_PRINT; }
-reference { return TOKEN_REFERENCE; }
+reference { Error(*yylloc, "\"reference\" qualifier is no longer supported; "
+                           "please use C++-style '&' syntax for references "
+                           "instead."); }
 return { return TOKEN_RETURN; }
 soa { return TOKEN_SOA; }
+sizeof { return TOKEN_SIZEOF; }
 static { return TOKEN_STATIC; }
 struct { return TOKEN_STRUCT; }
 switch { return TOKEN_SWITCH; }
@@ -223,6 +226,7 @@ L?\"(\\.|[^\\"])*\" { lStringConst(yylval, yylloc); return TOKEN_STRING_LITERAL;
 "&=" { return TOKEN_AND_ASSIGN; }
 "^=" { return TOKEN_XOR_ASSIGN; }
 "|=" { return TOKEN_OR_ASSIGN; }
+"->" { return TOKEN_PTR_OP; }
 ";"             { return ';'; }
 ("{"|"<%")      { return '{'; }
 ("}"|"%>")      { return '}'; }
@@ -266,8 +270,6 @@ L?\"(\\.|[^\\"])*\" { lStringConst(yylval, yylloc); return TOKEN_STRING_LITERAL;
 
 %%
 
-/*sizeof { return TOKEN_SIZEOF; }*/
-/*"->" { return TOKEN_PTR_OP; }*/
 /*short { return TOKEN_SHORT; }*/
 /*long { return TOKEN_LONG; }*/
 /*signed { return TOKEN_SIGNED; }*/