Feature: Persistent language changes

[tinmarino]

Fix 2/3 of #85

1. OK: Slang persistent
2. OK: Operator persistent (I changed the doc in the PR)
3. NO: Pragma persistent.

The first commit (81f5cfd) is the real work (11 lines) the rest is only test and doc.

[bduggan]

This is great! I wonder if this could be used to fix the issue in the built-in repl.

[tinmarino]

I think so, I'll see the issue. They already tweak the code to get the prompt with $code = $code ~ $newcode ~ "\n"; in the eval loop.