support for Plain TeX macros

[jarnowy]

Hello! New to tectonic, and already enthusiastic about it...

Speaking from the user base (I am no developer, though I hack bits and pieces here and there), I wonder whether is it planned to support Plain TeX macro packages, or whether is it feasible to support them. I am increasingly worried about the size/complexity of LaTeX2e+, which might be too much for most of my needs.

Ready to provide some testing, if not much else.