MergeStructs / MergeStructInto should support overwriting destination values

MergeStructs / MergeStructInto should support overwriting destination values: a task in MiMo-V2.6-RL-oss: Agentic RL Environments (MiMo RL release). MergeStructs / MergeStructInto should support overwriting destination values I'm using ygot.MergeStructs (and MergeStructInto) to combine two…