Rename reverse functions

[dpsanders]

We could rename the reverse functions to something like

rev(+) instead of plus_rev

[dpsanders]

inv would be another option but then we would be committing type piracy when we defined inv(+).

[dpsanders]

Another option would be a type like Reverse{+}. This is probably the best option.

cc @mewilhel

[mewilhel]

I like the Reverse{+} option. I think it'll make for neater definitions e.g. function Reverse{+}(...) as opposed to function rev(::typeof(+), ...). That said both seem like good options.