Use a Merge function, to make merging of >2 inputs clearer

[j16r]

This is a small change to JSON#Merge which lifts it out into a function, rationale:

1. As an isolated function it's clearer (to me) that this returns a copy and doesn't mutate anything
2. The original used Map() which potentially returns a the underlying data structure of the object, which could mean Merge actually mutates the caller if a resize happens.
3. Merge is now variadic, so merging in the RunManager becomes a bit clearer: models.Merge(previousTaskInput, currentTaskRun.Result.Data, run.Overrides) which makes it more obvious what is happening.
4. Reduced error checking, only turning the result back into gjson encounters an error signature, which only needs to happen once for any number of merge operations.

[coventry]

LGTM