Literate programming

[petithug]

Notes:

• I left Make Lemma alone, as it can be lifted as a top-level function or as a local (where clause) function, but the indentation would be different.
• The unlit function is fast, even on the 2600+ lines document I use.
• The unlit function returns a list of error lines, but only the first one is converted a error. But the code is ready for when the lexer and the parser start to generate multiple errors instead of just one.

Changelog:

• Add support for .lblod files.
• Add support for Case Split.
• Add new Parse Error for Literate fails.
• Add support for Add Clause.
• Literate processing fails if code blocks are not separated by a blank
  line.