Process library tracking issue

[marzipankaiser]

After merging #1146, there is now a minimal low-level process API. There are still things to do:

• The API is relatively low-level and un-Effekt-y. We should build a nice interface on top.
  • In particular, the current API uses lots of boxed callbacks that always need indirections via global state or similar.
• spawning a process currently uses a mutable builder for the options it gets passed (see Minimal subprocess API #1146 (comment) ). We could come up with a better interface here.
• Some cases (e.g. Minimal subprocess API #1146 (comment) ) still need handling for certain kinds of errors.
• Handle signals (Minimal subprocess API #1146 (comment) )