Reduce memory usage with sharing

[lehins]

Description

Fixes #3486

It will probably be easier to review it by commit rather than PR as a whole. Especially for changes to the GOV rule

Memory benchmark

Here is run of a memory benchmark from the ledger-state package of loading a snapshot from preprod network

Before this PR:

Case                     Max          MaxOS           Live       Allocated    GCs
NewEpochState  1,136,825,184  2,834,300,928  1,136,825,184  24,981,680,224  5,816

With deserialization sharing implemented in this PR:

Case                   Max          MaxOS         Live       Allocated    GCs
NewEpochState  993,573,512  2,598,371,328  993,573,512  25,151,778,496  5,857

This benchamrk serves as confirmation that sharing works as expected during deserialization. Once we have integration ready for cardano-node-10.3 we'll be able to test sharing during chain replay as well.

Checklist

• Commits in meaningful sequence and with useful messages
• Tests added or updated when needed
• CHANGELOG.md files updated for packages with externally visible changes
  
  New section is never added with the code changes. (See RELEASING.md)
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary
  
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code formatted (use scripts/fourmolize.sh)
• Cabal files formatted (use scripts/cabal-format.sh)
• hie.yaml updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[teodanciu]

Looks good to me.

I'm a little confused by the choice of implementing decSharePlusCBOR vs decShareCBOR, but that's probably more because my unfamiliarity with this DecShareCBOR typeclass.

What I gather is something like this:

• If you define getShare and decShareCBOR for a type, then if from "above" someone is calling decSharePlusCBOR for this type, then the getShare gets added to the shared state.
• If you define decSharePlusCBOR instead, then it's up to you what you add to the shared state - but if you don't explicitly put anything, is there actually anything added to it?

Maybe we could discuss about this when we talk, no need to waste time writing!