Rust-analyzer: Add CodeLens for references

[arzg]

Currently, the only CodeLenses that rust-analyzer shows are Run above fn main(), Run Test | Debug above tests, and n implementations above struct, enum, union and trait definitions. A useful addition to this would be a CodeLens of the form n references above definitions where this makes sense. When clicked, this CodeLens would show all the references to the symbol the definition defines.

Here’s a list of some possible definitions above which the references CodeLens could be shown:

• structs
• enums
• unions
• traits
• free functions
• functions in impl blocks
• functions in trait definitions

It _might_ be a good idea to show the CodeLens above enum variant definitions as well as struct and union field definitions, but that might be a bit much.

[vsrs]

I'm working on it.