Substantial rewrite (again) of decl handling.

The decl.* code now no longer interacts with Symbols, but just returns
names, types, initializer expressions, etc., as needed.  This makes the
code a bit more understandable.

Fixes issues #171 and #130.

tests/global-decl-define.ispc

@@ -0,0 +1,14 @@
+
+
+export uniform int width() { return programCount; }
+
+extern int foo;
+int foo = 1;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = foo; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}