Re-implement rustbook in terms of mdbook

mdbook has a lot of optional dependencies that we don't want, so instead
of using it directly, we re-build rustbook to use mdbook as a library.
For convenience' sake, we keep the same CLI interface as mdbook; the
only difference is that it only accepts build and test subcommands,
rather than the full range.

src/SUMMARY.md

@@ -0,0 +1,3 @@
+# Summary
+
+- [Chapter 1](./chapter_1.md)

src/chapter_1.md

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+# Chapter 1