Removing STMT_TUPLE_ASSIGN

[Martin Sjölund]

Tuple assignments have a problem today:

{{{class B

function fn

output Integer o1 := 1;
output Integer o2 := 2;
annotation(OpenModelica_Impure = true);

end fn;
Integer i1,i2;
Real r1,r2;

algorithm

(i1,i2) := fn();
(r1,r2) := fn();

end B;}}}

Class B would accept (r1,r2) := fn(), which works in this particular case (because in the C-code, a double can be assigned an integer at the loss of precision (>48-bit long ints; not a big issue)).
The big problem here is that converting an integer array to real array or unboxing returned data is not performed. We could either remove STMT_TUPLE_ASSIGN altogether (replace it with pattern assignments - my favorite) or extend the type to include a list of types so that the code generation can perform necessary casts but this does not always work.

We could also make the STMT_TUPLE_ASSIGN statement return a temporary variable plus a list of (non-tuple) assignments performed on this temp.var.

For now, I'll change the implementation to only check for subtype instead of type conversions, see if the testsuite runs, and commit.