Support for strings

[lhchavez]

The current plan is to expose them as a length/char[] pair in C, and a native string in the rest of the languages.

[gabrc52]

I wanted to upload this problem https://ioi2018.jp/wp-content/tasks/contest1/combo.pdf to OmegaUp, but I'm getting:

        int press(string p);

                  ^
``)'' expected but `string' found (line 2:12)

[lhchavez]

that's because this issue is still open (we still don't support string).