Support for Pointer and Reference types

[ghaith]

Is your feature request related to a problem? Please describe.
The standard supports a REF_TO type marking a reference
Some IEC61131-3 Extensions like Codesys support a POINTER TO type
This feature is to add support for these types.

Describe the solution you'd like
Add a new Datatype: POINTER TO <TYPE> where TYPE is any available datatype
Add a new Datatype: REF_TO <TYPE> where TYPE is any available datatype
Accessing a datatype instance as &instance will be a reference to that type.
Accessing a pointer or reference to a type using instance^ will de-reference the type.

Pointers and references are equivalent / Interchangeable.
Pointers and References can be assigned to a DWORD which is the address of the value being referenced.

Additional context
A pointer or reference that have not been assigned have the value NULL
Accessing a NULL pointer or reference is undefined

The standard calls for a REF() function, and codesys also provides an ADR() function to retrieve the pointer address. These functions are not covered by this feature request.