Avoid panic if request position is out of mapper's range

[buzztaiki]

Sometimes Emacs lsp-mode sends a request containing a position parameter before synchronizing with DidChangeTextDocument.
At that time, the mapper may not be able to find the stored position and panicked with unwrap.
This fix is to return Ok(None) instead of unwrap to avoid this case.

Perhaps this may fix the problem like a golang/go#48249 as well.

This PR includes commit of #184.

[tamasfe]

LGTM, thank you! This might fix the seemingly random panics such as reported in #142.

And sorry about the slow response.