feat! parse Markdown input with pull-cmark instead of Pandoc

This is a first step towards being able to use Subplot codegen from a
project's build.rs, and with only pure-Rust build dependencies.

Replace Pandoc for parsing Markdown input with pulldown-cmark. This is
mostly a drop-in replacement, but not entirely. The YAML parsing is
more strict now.

Note that this is a breaking change. Some subplots that used to work,
and still work with docgen, no longer work with the new parser. Major
differences are:

* Only specific fields are supported. All the Markdown files in the
  Subplot source tree work. If anything else is needed, and it likely
  is, the new parser needs to be extended.
* The bindings, functions, classes, and bibliography fields MUST be
  lists of strings. A single string value will no longer work.

Sponsored-by: pep.foundation

1 files changed, 17 insertions, 19 deletions

diff --git a/subplot.md b/subplot.md
index 83fbda3..824e44a 100644
--- a/subplot.md
+++ b/subplot.md
@@ -1,3 +1,20 @@
+---
+title: "Subplot"
+author: The Subplot project
+template: python
+bindings:
+- subplot.yaml
+- lib/runcmd.yaml
+- lib/files.yaml
+functions:
+- subplot.py
+- lib/files.py
+- lib/runcmd.py
+classes:
+- json
+...
+
+
 # Introduction
 
 Subplot is software to help capture and communicate acceptance
@@ -3153,22 +3170,3 @@ This is a test file.
 ~~~{#expected.txt .file}
 This is a test file.
 ~~~
-
-
-
-
----
-title: "Subplot"
-author: The Subplot project
-template: python
-bindings: 
-- subplot.yaml
-- lib/runcmd.yaml
-- lib/files.yaml
-functions: 
-- subplot.py
-- lib/files.py
-- lib/runcmd.py
-classes:
-- json
-...