Prepare tests for TeX and Metafont support
Rewrite the TeX MF framework
Remove unused option
Isolate support for METAPOST
Support stand-alone METAPOST documents
Validate the test suite for TeX and METAPOST support
Reorganise TeX document production

Use the _TEX_DOCUMENT variable to store radical of TeX documents.
Set TEXINPUTS and MPINPUTS when an object directory is active
Reorganise METAPOST document production

Use the _MPOST_DOCUMENT variable to store radical of METAPOST documents.
Narrow down the clean-files list
Subscribe log files to the clean-files list
Simplify the preparation of the clean-files list
Rename metapost.doc.mk to mpost.doc.mk

The newname is coherent with the command name and variable names
used in our module interface.
Reimplement the interaction option in the USES framework
Use a multipass strategy based on file cookies
Add support for bibtex
Ensure that cookies are newer than TeX products
Rename testsuite for bibtex to a more general name
Add support for index in LaTeX documents

Closes #110
Refactor build rules