Extract stage that emits symbols from lowering stage

At this moment there's a ton of problems having thins like `AddLabel` inside `Lower` stage, so we should at any cost move that thing into separate stage.

Simplest proposal is to just make a new method called `EmitSymbols`, which should be called in between or `Lower` and `EmitTo`.

There's naming issue - maybe EmitSymbols is not the best name, and naming it like that makes `EmitTo` sound inappropriate, so we should rename that as well.

I can submit PR for that if no one will complain

Start by tracing the Lower and EmitTo stages and locating where AddLabel is handled. Review how these stages are called and determine the intended boundary for symbol emission. Done means symbol emission is separated from lowering and the stage and method names are consistent, with existing behavior preserved.

Written by the indexing model from the issue text.