remove type aliases

[vinniefalco]

we should get rid of our aliases for error_code, result, et. al. and just use them directly from system.

[grisumbras]

Well, they make code smaller a little bit. Do they create a problem?

[vinniefalco]

It just can be a little confusing, documentation-wise. And when you are using multiple libraries that are in the habit of creating aliases for error_code, result, and the rest, now you have a lot of aliases...

[grisumbras]

There's an insane amount of projects on GH that use json::error_code. I feel like removing the alises will cause unnecessary chaos.

[vinniefalco]

Interesting, did not know that

[grisumbras]

For reference: boostorg/url#743.

The appropriate action is to deprecate aliases first, then remove after a few releases.