Fallible actions support

[dkumsh]

As discussed at #44

Fixes #44

This PR has 3 commits:

• add fallible actions support
• disallow guards mutating context/state (&mut self -> &self)
• rename GuardError to TypeError as it is now used not only by the guards, but also by the actions

[ryan-summers]

@dkumsh I'm refactoring internal state management in #76 to prevent the "State is None problem, which will simplify this a lot. If you want to wait until that PR gets merged and then rebase/merge, that would probably be easiest.

[dkumsh]

@dkumsh I'm refactoring internal state management in #76 to prevent the "State is None problem, which will simplify this a lot. If you want to wait until that PR gets merged and then rebase/merge, that would probably be easiest.

Thanks, totally makes sense. Will wait. Please let me know.

[ryan-summers]

Alright, #75 is now merged, so feel free to integrate that into your PR :)

[ryan-summers]

@dkumsh Did you intentionally close this PR? I think the changes would still be nice :)

[dkumsh]

@ryan-summers I think I closed it unintentionally.

I am re-opening this PR after rebasing on master.

As we agreed on the guards, I keep the change as well.

[ryan-summers]

Awesome, thanks!