fix mutations overriding spell level

[KorGgenT]

Summary

SUMMARY: Bugfixes "Fix mutation-learned spells overriding spell level upon load"

Purpose of change

Brought to my attention here:
chaosvolt/cdda-arcana-mod#82

Spells learned from a mutation were loaded in before the spellbook, because of on_mutation_gain adding the spell before the spellbook loaded in. Due to the spellbook being a map and the load spell using emplace(), the xp saved did not overwrite the original spell given by the mutation.

Describe the solution

Check if the spell is already known when loading the spell, and overwrite the xp if known. adds a new set_exp() function to spell to accomplish this

Additional context