[6.0] GetVar(inputIndex, varId) for reading context variable from another input and fix for Context.getVar

[kushti]

This PR contains:

• fix for CONTEXT.getVar failing deserialization and compilation ( close #978 )

• CONTEXT.getVarFromInput method to read context variable from any input.

  Note, that the new method is intentionally not throwing an exception on expected vs real types. This can be used to have getVar alternative without an exception, as in:

     {
         val idx = CONTEXT.selfBoxIndex
         sigmaProp(CONTEXT.getVarFromInput[Int](idx.toShort, 1.toByte).get == 5)
     }

[kushti]

@aslesarenko comments addressed , please make another pass