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-3.ispc

@@ -14,7 +14,7 @@ static float bar(float a, float b) {
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     float b = aFOO[0]-1;
-    FuncType func = foo;
+    FuncType func = &foo;
     if (a == 2)
         func = NULL;
     if (func != NULL)