serde support

Hello

Would you be supportive of a `serde` flag so that an entire `Blockstore` (ie `InMemoryBlockstore`) could also serialized?

So we can transport that entire Blockstore from memory to elsewhere too, if desired.

Dashmap does already have a `serde` feature flag.

Start by locating Blockstore and InMemoryBlockstore, then inspect the project's feature definitions and DashMap's serde support. Done means the serde feature allows an entire in-memory Blockstore to be serialized and transported elsewhere, with deserialization behavior covered by tests.

Written by the indexing model from the issue text.