Support for impure higher order function arguments

[pascutto]

Regarding pure clauses, I know about them, and I know about the implicit hypothesis that every higher-order function is pure (and requires pure arguments), but I would like to remove this hypothesis. Hence my suggestion to view isPure as a logical proposition, allowing a higher-order function (such as map) to explicitly indicate that its argument f must be pure (if desired).

Originally posted by @fpottier in #147 (comment)