Escape unescape

Created by: AgustinCB

I removed some things from the template that looked like would contain duplicated answers or no answer at all.

Problem: Need a way to unescape strings, similar to $"\n".

Solution: I created two new methods, escape and unescape.

Drawbacks:

• The logic about how to unescape is repeated in the method and in the builtin echo. I wonder if it's possible to dedup that.
• No tests. I didn't know how to write them

TODOs: [what is not done yet.]

Fixes: #556 (closed)

State: Ready. I guess. More like: Waiting for feedback :).