["Request"] StackSafeMonad

[ferranpujolcamins]

Description

Add a StackSafeMonad subclass of Monad, and move requirement for tailRecM from Monad to StackSafeMonad.

Why?
If I'm writing a new custom Monad, tailRecM is usually the trickiest part to implement. By separating it in another typeclass I can start by writing an instance for Monad and leave tailRecM for later.

It can also be helpful for people that are trying to get their head around Monad for the first time: they will be able to first learn how to implement and use flatMap and leave the trickier tailRecM for later.

Counter arguments
Scala's Cats mantainers chose to keep both tailRecM and flatMap requirements for Monad.
One argument was that they didn't want to have every method currently using tailRecM duplicated in a stack safe and a non stack safe version. I don't think this is a big problem here, since in Bow we only use tailRecM in Free.foldMap, and we can just require a StackSafeMonad instance. Also, it's ok to not offer non stack safe versions of those methods, at the end, the goal of this PR is not add support for monads that cannot be made tail recursive, but to allow quicker prototyping and learning by allowing users of the library to implement tailRecM later. We kinda "expect" that tailRecM will be implement anyway at some point in time to leverage all the functionality on a monad you write.

Another argument they make is that people might use flatMap for recursion because they don't have tailRecM available. But people might as well use flatMap even when tailRecM is available! In my opinion, what we need to do is add good docs explaining the issue with flatMap based recursion, rather than trying to "protect" our users.

Modules

Bow

Breaking changes

All current Monads will be StackSafeMonad, they'll have tailRecM but no explicit conformance to StackSafeMonad. The only thing that will brake until a StackSafeMonad conformance is added for a particular type is the tailRecM shortcut in Kind.

[ferranpujolcamins]

@truizlop This is quite an opinionated proposal, I'd like to know what do you think. Please feel free to disagree and close!