Generalize/improve parsing of pointer declarations.

Substantial improvements and generalizations to the parsing and
declaration handling code to properly parse declarations involving
pointers.  (No change to user-visible functionality, but this
lays groundwork for supporting a more general pointer model.)

tests/test-55.ispc

@@ -5,6 +5,6 @@ export uniform int width() { return programCount; }
 export void f_v(uniform float RET[]) { RET[programIndex] = 1.; } 
 export void f_v(uniform float RET[]) { RET[programIndex] = 2.; }
 
-export result(uniform float RET[]) {
+export void result(uniform float RET[]) {
     RET[programIndex] = 1.000000;
 }