Allow lifting of n-ary functions

[milibopp]

There is only lift2 right now. It is probably possible to do this generically, so that there is one generic function lift deprecating lift2. This will likely involve using a recursive macro to implement it for a finite but arbitrary number of arguments. Quickcheck has done something similar.

• Generic lifting for a few low arities (say, up to 4)
• Use a completely arbitrary arity

[milibopp]

#22 resolves the most common practical concerns, but is far from a general solution. However, it appears that Rust's type system can not accomodate such a solution, until it gets variadic generics.

[milibopp]

There was an idea on Reddit

[milibopp]

The situation here has not changed after the internals redesign.