Add implicit canonicalization for pushing a stream to a stream of collections

[alari]

One of the common patterns in Aqua is building optional values by writing or not writing to a stream:

func toOpt(value: string, flag: bool) -> ?string:
  res: *string
  if flag:
    res <<- value
  <- res

Consider the following snippet:

res_accum: *?string
res_accum <- toOpt("a", true)
res_accum <- toOpt("b", false)
res_accum <- nil

In this case, res_accum is defined as a stream of optional values. Finally it compiles to smth like this:

(match true true
   (ap "a" $res)
)
(ap $res $res_accum)

This code will fail, as $res value is not deterministic: different peers may observe different state of $res. $res, being a stream, forms a locally consistent CRDT, its state may differ for different points of view, and may grow in time.

To use $res safely as a value of type ?string (the return type of the function), it needs to be canonicalized: converted from the mutable $res form to an immutable res form.

Canonicalization is not yet implemented on AquaVM side, but we can think of it as a point of observation:

(can peer_id $res res)

After this can call, res will contain a fixed value of $res for the moment of the first can call on peer_id. All other peers will check that res was derived the right way, get tetraplets, etc.

Until we have this implemented, we can mock can with (call peer_id ("op" "identity") [$res] res). Currently, it does the same: converts argument to a scalar value. But it misses tetraplets.

The task is to add this call implicitly when a stream is pushed into a stream. Once we have native canonicalization support, replace op identity call with it.