`Igniter.Refactors.Rename.rename_function/3` should update the `@spec` associated with the definition

Something I noticed while testing a `rename_function` call. It will upgrade the function name at the definition site, but not the `@spec` preceding it, so renaming `foo/2` to `bar/2` will get you something like:

```elixir
@spec foo(term(), term()) :: term()
def bar(x, y) do
...
end
```

Start at Igniter.Refactors.Rename.rename_function/3 and trace how the function definition is renamed, then inspect how the preceding @spec is represented. Done means renaming foo/2 to bar/2 also changes the associated spec name while preserving its arguments and return type.

Written by the indexing model from the issue text.