Make `Filters` constructible from `IntoIterator`s

[david-perez]

This just relaxes bounds. Useful to create filters from arrays.

[mitsuhiko]

I am in favor of this change, but I'm a bit worried about now having a generic T implementation there. It might be okay, but then it cannot be implemented for potentially other things in the future. I will think about this change a bit more.

[david-perez]

Ah yeah you're right. I've pushed a commit to implement FromIterator instead. I've also restored the From<Vec<(&'a str, &'a str)>> implementation, since removing it is a breaking change, but there shouldn't be a need for it if we have FromIterator.

[mitsuhiko]

Applied in 1d702385e13cdfc2bf81616c7b4b349501856c12