Use macro loops to implement traits

[phimuemue]

I saw the comment at https://github.com/csherratt/tuple_utils/blob/master/src/lib.rs#L129 and thought that we could replace all those manual impls with ones created by macro loops.

Basically, I delegated the work to two macros:

• Essentailly, for_each_prefix takes a macro and a list of arguments and applies the macro to all prefixes of the argument list.
• for_each_prefix_suffix does something similar, but applies the given macro to the suffixes, too.

For simplicity, I always used trailing commas in the macros. In addition, I replaced type names A, B, C... with T0, T1, T2....

[csherratt]

Thanks! Sorry I missed this pull request and let it sit for so long. It's appreciated.

[phimuemue]

No worries!