342 add a debug mode

[daniel-raffler]

Adds a debug mode that helps find common user errors

• We check that solver structures are only accessed from the thread that created them
• We make sure that formulas are not used outside of their context
• Configuration options can be used to enable/disable the checks

This is only a draft. Please don't merge yet.

EDIT: Added a patch for testing: https://gist.github.com/daniel-raffler/6bfc3044232ea1cd542f43d9442f5495 This will run all JavaSMT tests in debug mode