Refactor ActionT to use ReaderT pattern

[ocramz]

scotty-0.20 :

• Rewrite ActionT using the "ReaderT pattern" : https://www.fpcomplete.com/blog/readert-design-pattern/
• Clarify the exception handling mechanism of ActionT, ScottyT. The StatusError type can be thrown and caught, but Next, Finish and Redirect cannot be caught by the user. For StatusError and any user-defined (or imported) exception types, one can either catch exceptions inline with rescue or globally for the whole app with defaultHandler.

Breaking changes:

• Introduce unliftio as a dependency, and base exception handling on catch.
• rescue changes signature to use proper Exception types rather than strings.
• All ActionT methods (text, html etc.) have now a MonadIO constraint on the base monad rather than Monad because the Response is in a TVar inside ActionEnv. rescue has a MonadUnliftIO constraint. The Alternative instance of ActionT is also based on MonadUnliftIO because <|> is implemented in terms of catch.

Closes #310 , closes #309 in passing

closes #110 , closes #153

[ocramz]

it works omg it w o r k s I'm so happeeeehhh

[fumieval]

Noice

[fumieval]

@ocramz Is it intentional that you merged this PR? Before merging this PR, At very least I expected you to

• Clean up fixme/WIP commits by git rebase I see the commits squashed on master, that's great
• Wait for other maintainers' approval

[ocramz]

@fumieval yes sorry for not waiting but I triple checked everything, added more tests around the new exception mechanism and made a Hackage release. These were long-due changes anyway..