First version of initializer support for 'to' block

[agarciadom]

Fixes #125.

I had a first try at supporting initializer expressions in the to part of an ETL rule. I've tried implementing the change in the grammar, unparser, default strategy, and fast strategy, and created a test around this transformation:

rule Game2Game
  transform s: Points!points::Game
  to t: Coins!coins::Game
{
  t.coins ::= s.scores;
}

rule Score2Coin
  transform s: Points!PointScore
  to c: Coins!Coin = s.score > 5000 ? new Coins!GoldCoin : new Coins!SilverCoin
{
  // nothing to do
}

Let me know if I've missed anything :-).

[agarciadom]

Looks like I'm failing some tests. I'll work a bit more on this PR later.

[agarciadom]

The PR is now passing all tests - @kolovos can you check it?

[kolovos]

Looks good to me!