vim file improvements

`colorcolumn` is now set with `setlocal`, so it doesn't infect the entire session.

`conceal` support is now opt-in, not opt-out. I haven't found anybody that likes it or cares either way.

The indent file now uses `cindent` with appropriate `cinoptions` and `cinkeys`. Auto-indenting an entire file mostly gets the right thing now, with structs and enums not being affects like before. Defaults to the hanging indent style, unless the first argument is on a new line.

There were some keywords in the syntax file that I've never seen (and have been used as identifiers in some tests) so they have been commented out.