Improve load time on serialized entries.
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From profiling slow load times of serialized GPUv2 delegate data, a lot of unnecessary time is spent re-allocating memory as part of the call to std::string::append. By pre-allocating the correct size, we save a lot of time.

This patch was developed in 2022 (pardon the slow turn-around) and I no longer have the exact profiling details. However, my notes say that on an arbitrary but complex model, GetData() was previously taking 50ms; with this patch, it takes 18ms.