vader#log: fix self-refs and handling of partials with Neovim

When using `Log` / `vader#log` you might run into issues if `string()` fails on the object, especially with Neovim.

In Neomake I have a helper function for this, which would make sense to use with `vader#log` in Vader already likely.

https://github.com/neomake/neomake/blob/901f97a96796ef44ef1ca465c56784d1bb8292fb/autoload/neomake/utils.vim#L605-L644

Followup: https://github.com/neomake/neomake/pull/2464

Ref: https://github.com/neovim/neovim/issues/7432