Allow struct variables to be passed directly as struct type instead of ADDRESS

[mdbergmann]

Now we have to go a detour over ADDRESS to pass structs:

DECLARE STRUCT Bar abc

SUB foo(ADDRESS barAddr)
DECLARE STRUCT *bar
  bar = barAddr
END SUB

The new addition should allow passing structs directly:

DECLARE STRUCT Bar abc

SUB foo(Bar bar)
END SUB

But this will just be syntactic sugar over the ADDRESS detour.
Full typed METHODs with CLASSes comes next.

[mdbergmann]

implemented with #95