Highlight unused / duplicate service clauses

[kubukoz]

Examples:

• multiple references to the same service in the prelude
• (to be considered) explicit clause matches existing use clause
• no operations refer to an imported service

This will probably happen in something like PreludeCompiler, but will need some extras for the operations' explicit service clauses.

Possibly in scope:

• provide code actions to remove these
• hook into "unused imports" text editor command (might be a vscode-only thing, so that'd be a custom LSP request)

happens after #126 is merged