rename /x/ to /extensions or /modules

[ValarDragon]

We had some internal discussion on this quite awhile ago. I think x is unclear (at minimum its unclear that it stands for extensions within our docs). This just feels like an odd barrier to entry to have. This is also done on IRIS' repo, and I think it looks a lot cleaner. https://github.com/irisnet/irishub

[jaekwon]

They're all modules, and /extensions is too long. Golang uses /x/.
After a while (e.g. a year, after stability) we can move them out into / where name isn't extensions or modules.

[ValarDragon]

Good point!