Rename vim-syntax-keywords -> vim-keywords
Extract vim interaction to vim-exec
Add syntax performance benchmarks

With frak fork at:

    https://github.com/guns/frak/tree/public-escape-chars

Testing reveals that `syntax keyword` is ~2x faster than an equivalent
frak-optimized `syntax match` for the same words, and ~8x faster than a
naive approach using `syntax match`. Subjective experience matches these
results.

Clojure syntax highlighting is already a bit slow, so a 2x performance
hit is not really acceptable.