Use `nvim_set_hl()` when appropriate

[wincent]

This isn't a drop-in replacement for :highlight, but I expect it should be faster. Will want to benchmark that and see.

[wincent]

Closed by:

• https://github.com/wincent/base16-nvim/commit/e71ae2694402d5e9227228d66ac273e5a5a2af76