Handle complex assignment

[guillaumeduhamel]

tests/proposal/assign_*.arl

for example:
tests/proposal/assign_record.arl

archetype assign_record

record myrecord {
  x : nat;
  y : string;
  z : int;
}

variable v : myrecord = {0; ""; 0i}

entry exec () {
  v.y := "str";
  const ref : myrecord = {0; "str"; 0i};
  do_require(v = ref, "INVALID_VALUE")
}