A few API inconsistencies around Guard reuse and the default hasher

A few API inconsistencies around Guard reuse and the default hasher: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). I've been using flurry::HashMap in a multi-threaded service and ran into two papercuts that feel like they should be fixed together.