Clean up EquationSearch with a SearchContext struct

[MilesCranmer]

One of the barriers to simplifying EquationSearch is that there are many variables that need to be passed around. Therefore I think it would be great if there was a single SearchContext struct so that all of these key variables could be stored.

Ideally I would like for EquationSearch to simply be a list of steps, like:

fetch_process(search_context, process_context[i], options)
update_hall_of_fame(search_context, options)
write_hall_of_fame_to_disk(search_context, options)
migration(search_context, options)
spawn_process(search_context, process_context[i], options)