parse the canonical prelude, fixes #93

The original and canonical prelude can differ. For example, if the
original doesn't import Prelude (and doesn't use NoImportPrelude), the
canonical prelude will add the import. But since we were parsing the
original, the user expression could no use the prelude.

Calling `extendModules` might have been faster, though.
enable the last test of the documentation

If you have attempted to run this test before (unlikely since it has
always been commented-out), you will have to remove its cache folder:

  doc/conversions/cache
Merge branch 'issue-93' into develop