preliminary support for multi-arg map and mapreduce with breaking changes

[ExpandingMan]

This adds support for multi-argument map and includes breaking API changes for both map and mapreduce, as discussed in #54. The main motivation is for multi-argument mapreduce; that hasn't been added in this PR, but considering that I am proposing breaking changes I wanted to start a discussion before I decide how to do it. If this is merged (whether with the API I'm proposing or some suggested alternative) I will make another PR with multi-argument mapreduce.

There are several alternatives to what I'm proposing here that are non-breaking, including

• Wrapper type for multi-arg (though the natural candidate Tuple would seem like a bad idea because Tuple is itself iterable and can be used in Base.map).
• New function for multi-arg.
• Pass additional arrays via their own keyword argument.

However, I would find this to be a pretty compelling case for a breaking change since it would make this package consistent with Base and also other similar packages such as OhMyThreads.jl.

Note that I have not moved backend to a keyword argument in the various arithmetic functions since there isn't nearly as strong a case that they should be changed, though, for what it's worth, if it were up to me, I'd probably change those also.

[ExpandingMan]

This seems to fail on metal, but I don't know why. Everything works fine on me on CPU and with CUDA.