Request: unused variable analyzer

There is already present `unused local variable` in Roslyn, but it works only for const values, which makes it almost useless.

Would be really great to have warning, if there is some local variable declared and unused.
If we need to execute some method, but don't care about result - we shouldn't assign returned values at all.
Edge case - we need to execute property, but don't care about result - makes sense to use underscore variable name (therefore analyzer should skip vars with `_` name).