Low-level reflection API for setting position of generated terms.

[rssh]

Need to expose setPos to the macro level.
Typical situation - when we have macro wrapped some chunk of code.

chunk {
  expr1,
  expr2,
  expr3
}

which is transformed into something generated at the top-level: generation(expr1, expr2, expr3).
then when we try to retype the received expression and receive an error, our error is always the beginning of the chunk.

With the ability to set the position of generated symbols, the author of macro libraries will provide better diagnostics.