Implement a parser for a literate testing system for the repl

This parser can be reused for other purposes. It's inspired by
https://bitheap.org/cram/

Although eelco's impostor exists https://github.com/mobusoperandi/eelco,
it is not very nice to depend on out of tree testing frameworks with no
way to customize them.

Change-Id: Ifca50177e09730182baf0ebf829c3505bbb0274a

tests/functional/repl_characterization/data/basic.ast

@@ -0,0 +1,11 @@
+Commentary "meow meow meow"
+Command "command"
+Output "output output one"
+Output ""
+Output ""
+Output "output output two"
+Commentary "meow meow"
+Command "command two"
+Output "output output output"
+Commentary "commentary"
+Output "output output output"