remove implicit Boolean to Fox conversion

[jstriebel]

Conceptionally false is a valid value that can be checked for. Converting this implicitly to a failed Fox makes some expressions to magical (e.g. resolving assertions via the Fox ?~> statement here.
Let's have a look how often this is used and if it makes sense to remove it / convert it to explicit conversions.

[jstriebel]

That also leads to that x ?~> … behaves differently on Boolean and Fox[Boolean], where Fox[Boolean] seems to be a legit construct.
(Which is similar to Fox[Option], but this does not seem legit to me, as Fox overrules Option and should therefore be converted instead of wrapped. Wrapping a boolean in a Fox is fine.)
Could & should we actually just forbid Fox[Option] and Fox[Future] by having type bounds on Fox itself? /cc @fm3

[fm3]

maybe, but that may create problems with function return types and intermediate values that are then flattened

[jstriebel]

one more argument: #2953