Follow-up of #3527 and IDEA discussion.

Currently, sequential coroutine builders are part of the debugger: their states are being tracked, and they can be seen in IDEA "Coroutine debugger" pane under "Empty dispatcher".

The code with such builders is short-lived, blazing fast (e.g. no real concurrency involved), and, in fact, can be used as a more stylish or idiomatic way to generate a plain old sequence/array/list.
In such scenarios, the overhead of DebugProbes becomes visible (efe80a1): depending on the workload, the behaviour of concurrently executed coroutines and timings, the performance of sequence builders might take up to 50% hit (in our IDEA's codebase it is around 25-30% and is important to the extent that code is rewritten from coroutine-based builders to iterators), becoming quite a non-zero cost abstraction.

Apart from that, the state of the sequence builder doesn't seem to add any value to the debugging experience -- it's always linear, non-concurrent, and cannot contribute to the system observability and/or progress (-> is never the source of the deadlock/livelock with coroutines).

Taking it into account, my suggestion is to explicitly filter them out in the debugger if it's possible