Stack usage optimization for tostream_visitor

[thehans]

This change was tested against the sample code in #4172, and while it does not fix the crash, it delays the crash until higher values of b, by going through fewer function calls, and re-using tostream_visitor instead of constructing new ones for each nested vector.

My testing showed the lowest value to trigger a crash went up by about 1000 after changes.
Lowest b value required to trigger a crash, built with g++-10

RelWithDebInfo build:
  unedited master: b=5390
  with changes:    b=6378
Release build:
  unedited master: b=4936
  with changes:    b=6081
Debug build:
  unedited master: b=3465
  with changes:    b=3905

Also removed some Value::toString|toStream functions which were not being used.

[MichaelPFrey]

#4173 contains a similar approach partially implemented. This implementation here is a lot cleaner.

As I want to discard #4173 anyway to get a cleaner implementation, merging this here is welcome by me.

I think fixing #4172 (add stack check for vectors to toEchoString()) will also be its own merge request (before show usermodule parameters in trace).

Splitting it up makes things a lot easier.

[thehans]

Thanks for looking it over, I was hoping you would see this since it affects some of the related code you were working on, and alters the required b value for the test case by a significant amount.