Add the go ctags parser

Make go one of the builtin filetypes, add the parser and update the related
source and config files. While there, remove Rust from [Groups] in
filetype_extensions.conf because it's already a builtin filetype as well.

The parser itself is stolen from the fishman/ctags repo.

The parser is more on the simple side - types like structs or interfaces are not distinguished (I may have a look at it in the future if I have time, should be easy to add) but it's definitely useful for going to tag definition/declaration.

I hope I have changed all that should have been changed and haven't forgotten about anything. Everything seems to work just fine, including syntax highlighting, build commands sidebar symbols, goto tag definition/declaration.
Who doesn't? Well, the code is a bit strange...