Mutations with Default Values

[llchan]

The C++ mutation methods fail if the buffer has a field set to the default value and it is asked to mutate it to the same default value. This happens because SetField<T>(field, val) looks for that offset and returns false when it's missing. I think we should pass in defaultval and return val == defaultval instead when the field is unset.

As a workaround, I can add a condition on the outside to check for equality. Long term though, I think it would be best to make this change internally, both for readability and for performance.

[llchan]

I should add that the behavior implied with the proposed change is that if the value goes from a non-default value back to the default, it should modify the existing field rather than invalidate the operation. The assumption is that mutation is used when you want to maximize reuse, so it should invalidate as infrequently as possible.

[ghost]

Agreed with both of those. Can you make a PR?