feat(nns): Avoid recomputing wasm/arg hashes during read operations

[jasonz-dfinity]

Why

Calculating hashes for wasm/arg actually takes quite a bit of instructions (80/byte), which makes it the dominant part of the list_proposals instructions. Such hashes can be calculated during write operations easily, since the data should be immutable.

What

• Add wasm_module_hash and arg_hash into InstallCode internal type
• Move hash calculation from read operation (internal -> response) to write operation (request -> internal)
• Add backfill for existing Installcode proposals
• Update benchmark results (96% improvement on list_proposals)