Currently there are a couple ways to set a field of a MutableDocument to a new value. The only one two that are move optimized are:

doc.getField(fieldName) = Value(val);
doc.getNestedField(fieldName) = Value(val);

We should remove all variants of MutableDocument::setField and MutableDocument::setNestedField in favor of the above strategy. We should also remove MutableDocument::getField in favor of MutableDocument::operator[]