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-6.ispc

@@ -16,7 +16,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     float b = aFOO[0]-1;
     FuncType func = NULL;
     if (a == 2)
-        func = foo;
+        func = &foo;
     if (!func)
         RET[programIndex] = -1;
     else