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.
I think it's more or less OK the way it is - as you say, the worst thing can happen is the extra semicolon which the parser just skips at top level.

Go has optional semicolons which are automatically inserted based on some simple rules:

https://golang.org/ref/spec#Semicolons

So some tracking of the last token is necessary.