Remove and refactor duplicate offset functions

[fatih]

Right now there are several places in vim-go that parses the byte offset of the position under the cursor. The ones I could see are below. And what interesting is that each of them has it's own way of implementing it:

https://github.com/fatih/vim-go/blob/master/autoload/go/complete.vim#L90
https://github.com/fatih/vim-go/blob/master/autoload/go/rename.vim#L67
https://github.com/fatih/vim-go/blob/master/autoload/go/oracle.vim#L65
https://github.com/fatih/vim-go/blob/master/autoload/go/def.vim#L67

Let us create a single function under util.vim with the function signature: go#util#OffSet(), remove all others and use this new function.

[nhooyr]

I'll work on this tomorrow.

[nhooyr]

The ones in oracle.vim and rename.vim are the exact same.