Support testing/clearing Assignable_Proxy_Function's callable

[totalgee]

Issue this pull request references: #321

Changes proposed in this pull request

• add operator bool() cast to test if an Assignable_Proxy_Function has a callable.
• add clear() to reset/empty the callable function.
• include new compiled test case to work with (setting, testing, calling and clearing) callback functions in C++ objects.

[totalgee]

Incidentally, when I run make test or just ctest locally, if there is actually an error it still shows the tests as passing. However if you actually run ./compiled_tests you'll see if there were any failing exceptions or CHECK() clauses. Why is that? Does "compiled_tests" not have the proper return code? Hopefully it actually checks these things properly when running the continuous integration, if not in my local build.