fix rendering of column by `srcloc->string`
at-exp reader: fix source locations in `read` error message

A change in `raise-reader-error` to use `srcloc->string` caused
tests to fail because `srcloc->string` doesn't show location
information if the source is #f.
Scribble->Markdown titles are now rendered correctly. Sections have been shifted by one level (which also looks better on Github).

Markdown: fix defmodule to avoid printing the package in the example

fix markdown example file to make test pass
enable scribble tests for DrDr