Don't copy mutation branches unecessarily

[anothersimulacrum]

Summary

Performance "Speed up navigation in traits menu"

Purpose of change

I've never experienced this menu to be terribly slow, but @KorGgenT has often complained about it in the past.
I profiled it and saw that checking if traits conflict via has_same_trait_type was taking a huge percentage of the time.

Describe the solution

My profiling showed that most of the overhead from has_same_trait_type was due to get_mutations_in_type, and most of that due to constructing mutations branches. So, stop taking uneccessary copies, and use references.

Describe alternatives you've considered

This is about a five minute fix, and perf is telling me it's a huge improvement, so I'm happy to leave it here.

Testing

perf record -g -p PID when in the traits menu, scroll around and select a few traits.

Additional context

Before:

After: