Decide how to handle serdect breaking change

[conradoplg]

serdect changed its encoding format in 0.3.0, which is breaking change for us.

Since we have a version field, we could introduce a new format version that uses 0.3.0, but we'd need to keep support for v0 format. This would involve a lot of code. So for now it seems better to simply keep using 0.2.0.

However, we might want to update if we determine that any of the reasons that caused serdect to upgrade are actually relevant to us (lower performance; possible variable timing in some cases, encoding inefficiency in some formats like MessagePack)