The immediate motivation for introducing frozen_mutation is inability
to deserialize current "mutation" object, which needs schema reference
at the time it's constructed. It needs schema to initialize its
internal maps with proper key comparators, which depend on schema.
frozen_mutation is an immutable, compact form of a mutation. It
doesn't use complex in-memory strucutres, data is stored in a linear
buffer. In case of frozen_mutation schema needs to be supplied only at
the time mutation partition is visited. Therefore it can be trivially
deserialized without schema.
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