Add identifier range info

[mgood]

In order to support Microsoft/vscode-go#962 this exposes the start/end range of the identifiers for each declaration. In some case (particularly imports) the ast.Ident is nil, so the identifier range is possibly nil too.

I've quickly checked the output of this on some Go files, but since it requires some more integration work on the vscode-go side, it's probably best to finish that before merging in case this patch needs any more work.