[6.0] Box.getReg implementation

[kushti]

In this PR, Box.getReg to get register contents by index is implemented

Example:

{
   val x = SELF
   x.getReg[Long](0).get == SELF.value &&
   x.getReg[Coll[(Coll[Byte], Long)]](2).get == SELF.tokens &&
   x.getReg[Int](9).isEmpty
}

close #416

[aslesarenko]

It is better to version this test as well.

[kushti]

done

[aslesarenko]

The idea behind getReg was to have "computable" index, not just constant.
For example the index can come from Context variable and thus the concrete register will not be fixed in the ErgoTree. This will allow code like:
val x = box.getReg[Int](getVar[Int](varId).get).get

So I suggest to add additional test with variable register passed as function argument and have a series of cases in verifyCases.
"{ (x: (Box, Int)) => x._1.getReg[Long](x._2).get }"

[kushti]

{ (x: (Box, Int)) => x._1.getReg[Long](x._2).get } is way different from box.getReg[Int](getVar[Int](varId).get).get ,
so I've made a test in BasicOpsSpecification which is checking exactly context variable usage:

{
   val x = SELF.getReg[Long](getVar[Int](1).get).get
   x == SELF.value
}

and so the new test can also be used for reference for people asked for computable index.

[kushti]

@aslesarenko comments addressed, please make another pass!