error was missing in secondary keywords (also sorted alphabetically now)
Add true false iota nil into primary keywords which, while not strictly
speaking keywords (they are predeclared identifiers), are nice to have
highlighted.
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.
