VariantBuilder (for initial review)

In this case, it will be fine. realloc() is only actually dangerous (ie., leads to undefined behavior) if it's called when we are using the static buffer in VariantBuilderDataField as our storage. We never use the static buffer when actually building the record, however (which is what we're doing here) -- this memory is always malloc()'d.

Agree that estimate_* is a better name for this function.