Streamline macro syntax

[huitseeker]

The Problem

The current macro system requires learning what 7 auxiliary macros do: zip_with, zip_with_for_each, zip_with_into_par_iter, zip_with_iter, zip_with_par_iter, zip_with_par_iter_mut_for_each, zip_with_into_iter.

This PR

Switches us to a mode in which the user has only 2 macros to learn about : zip_with and zip_with_for_each.
This further documents them as such:

// no iterator projection specified: the macro assumes the arguments *are* iterators
// zip_with!((iter1, iter2, iter3), |a, b, c| a + b + c) -> iter1.zip_eq(iter2.zip_eq(iter3)).map(|(a, (b, c))| a + b + c)
//
// iterator projection specified: use it on each argument
// zip_with!(par_iter, (vec1, vec2, vec3), |a, b, c| a + b + c) -> 
// vec1.par_iter().zip_eq(vec2.par_iter().zip_eq(vec3.par_iter())).map(|(a, (b, c))| a + b + c)

and:

// no iterator projection specified: the macro assumes the arguments *are* iterators
// zip_with_for_each!((iter1, iter2, iter3), |a, b, c| a + b + c) -> iter1.zip_eq(iter2.zip_eq(iter3)).for_each(|(a, (b, c))| a + b + c)
//
// iterator projection specified: use it on each argument
// zip_with_for_each!(par_iter, (vec1, vec2, vec3), |a, b, c| a + b + c) ->
//   vec1.par_iter().zip_eq(vec2.par_iter().zip_eq(vec3.par_iter())).for_each(|(a, (b, c))| a + b + c)

Everything is refactored to use one of these two macros. While there are more complex forms to these macros, the user doesn't need to import them or know about them. The implementations are also simplified.

Details

Also contains #170 because stacked PRs in Git are a mess.

[porcuquine]

This is a natural next (perhaps final) evolution of these macros.