Merge: Nitunit: mass compile non-simple docunits

Merge: Nitunit: mass compile non-simple docunits

When docunits are simple (just statements), a single module is generated for all.
Unfortunately, when docunits contains classes or importations, a specific module must be generated for each one of them.
Compiling all these module takes most of the nitunit elapsed time.

This PR tries to compile these non-simple modules at once, thanks to nitc that features invoking it with multiple programs.

Testing with `nitunit lib/popcorn/README.md`

Before:
* cold ccache: 1m3,832s
* hot ccache: 0m23,063s

After:
* cold ccache: 0m44,517s (-30%)
* hot ccache: 0m17,778s (-23%)

Pull-Request: #2510
Reviewed-by: Jean-Christophe Beaupré <jcbrinfo.public@gmail.com>