RFC: Rename `int/uint` to `intx/uintx`

This RFC proposes to rename the pointer-sized integer types `int/uint` to `intx/uintx`, and use new literal suffixes `ix/ux`, so as to avoid misconceptions and misuses.

This is yet another attempt to rename `int/uint`, and I believe the new names `intx/uintx` address the drawbacks of previously proposed alternatives. See [A tale of two’s complement](http://discuss.rust-lang.org/t/a-tale-of-twos-complement/1062) for reasons of the rejection of the previous proposal.

[Rendered View](https://github.com/CloudiDust/rfcs/blob/int-to-intx/text/0000-int-to-intx.md).