Draft: Toml config

First part of #224.

I was hoping to get this in for 2.7, but that's been delayed long enough.

I think merging the global and prefix-specific configs on the rust side was a mistake, due to how some profile variables are handled. I will change it so that they are merged on the python side in a way that can share code with the existing python config.

TODO:

• #224 (comment 1594508286) (profile symlink should be replaced by a path in portmod.toml)