feat: mark and sweep environment

[wxiaoyun]

Description

• Refactor all references to environments by thread, closures and runtime_stack to weak references
• Centralize ownership of environment to runtime
• Mark and sweep environment
  • For 1 second (or other user defined time), there is going to be a round of mark and sweep:
  • Mark: Recursively mark all environments reachable from all non-zombie threads
  • Sweep: De-allocate any unmarked environments
  • Since only the runtime has ownership of the environments, we can de-allocate any environments that are unreachable by dropping them in the Runtime struct
• Add support to anonymous functions

Bug fixes

• Previously, extend_environment extends the current current at all times. This is not correct as for function calls, we want to extend the environment of the function. This PR corrects this behavior.

Resolves #47

[leonidas1712]

Great work, thanks!