Revamp handling of function types, conversion to function ptr types.

Implicit conversion to function types is now a more standard part of
the type conversion infrastructure, rather than special cases of things
like FunctionSymbolExpr immediately returning a pointer type, etc.

Improved AddressOfExpr::TypeCheck() to actually issue errors in cases
where it's illegal to take the address of an expression.

Added AddressOfExpr::GetConstant() implementation that handles taking
the address of functions.

Issue #223.

tests/funcptr-null-1.ispc

@@ -15,7 +15,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     float b = aFOO[0]-1;
     uniform FuncType func = foo;
-    RET[programIndex] = (func ? func : bar)(a, b);
+    RET[programIndex] = (func ? func : &bar)(a, b);
 }
 
 export void result(uniform float RET[]) {