## Description 

Added compiler warnings for unused (uninstantiated) struct types and
unused (not called) private functions that were originally proposed as
linters in https://github.com/MystenLabs/sui/pull/11212

## Test Plan 

Will add tests

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### Type of Change (Check all that apply)

- [ ] protocol change
- [x] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

The compiler features additional warnings for unused private functions
and unused (uninstantiated) struct types.