Add support for running tests in documentation

[oubiwann]

One of the classic problems with putting code in documentation is that it, sooner or later, begins to bit rot. If code in documentation can be run as part of a test suite (e.g., prior to commits or as a gate in a continuous integration platform), then this can be checked (and hopefully prevented).

The Python programming language pioneered this with doctests. Though sometimes hailed as a great abuse of testing (which it can be), when used appropriately (e.g., ensuring that code in documentation continues to be checked as working) it can be a great boon to software projects which strive to provide good example usage of APIs, etc., in documentation.

I suspect this will actually be comprised of a sizable number of other tickets. The following tasks will likely need to be completed to make this a reality:

• Parse Markdown files for code blocks tagged with "cl" or "lisp"
  • possibly add doctest metadata and/or support for doctest directives in the first line of the code block
• Parse commented code in LFE modules - this will definitely need some sort of metadata marker indicating that the code to follow should be parsed by ltest's doctest mechanism
• Parse docstrings in LFE functions - I have doubts about this one; one can only put so much code in a docstring before it becomes awkward to read (much more to parse and execute as a test ...)
• Execute parsed code, line-by-line
• Ensure that executed code returns the same result as provided in the parsed code example/documentation
• Add testing infrastructure
  • Add a behaviour for "doctest"
  • Add a (defdoctest ...) macro
  • Add supporting helper functions for this macros which:
    • Add support for running all code in provided Markdown files (code that is marked as "doctest", that is)
    • Add support for running all code in the code comments of provided (or discovered) files
    • Add support for running all code in the docstrings of provided (or discovered) files (again, not sure about this one)
• Update test runner to include these types of test "suites"

[oubiwann]

Note that, as the number of Markdown-based docs continue to grow (e.g., the gitbook books), this -- or something like it -- becomes increasingly important.