[Enhancement] Add diagnostic on never instantiated entities

[pidgeon777]

Actually, when opening a VHDL entity which is not part of a project library, rust_hdl reports it.

It would be nice to also add a diagnostic message to the entities (and also their architectures for completeness) not instantiated in any of the other entities of the project.

The diagnostic level could be set to WARN, or maybe better INFO.

The diagnostic could be enabled or not, and its severity level set, by the user thanks to #162.

[kraigher]

Detecting unused code such as variables, signals, functions etc that are not read/written is something I hope to implement. There needs to be some side information that says a function or entity may be unused. For example if you are working on an IP-core library some entities might be unused within your own project and must be marked as "public"

[kraigher]

For signals and variables as well as local functions it is easier as those can never be accessed from the outside unlike entities and functions in package headers.