Pass `ctx` to context & Simplify callstring-based approaches

[michael-schwarz]

• [x] Pass ctx as additional argument to context
• [x] Simplify callstring-based approaches to not track callstring in local state
• [x] Simplify loopfree callstring
• [x] Investigate how this works with context failing for startvars

Closes #1379.

[michael-schwarz]

This changes the behavior for function calls somewhat, in the sense that new threads start with an empty callstring.