refactor: use nvim_set_hl to speed up startup time

[cljoly]

On my machine, prior to this change, the colorscheme adds about 8ms to
the startup time of neovim. Using the newly introduced API
nvim_set_hl, it’s a little under 3ms, so that’s more than a two times
speedup!

Special care is taken to handle the fmt argument, as that’s the least
friendly to nvim_set_hl.

This change strives to be backward compatible. Once neovim version 0.7.0
will be spread widely enough, further performance gain are within reach
by:

• removing the fallback that calls string.format
• defining the group_settings so that those can be passed to nvim_set_hl directly

[cljoly]

Using https://github.com/rhysd/vim-startuptime with vim-startuptime -vimpath nvim, before the change on my machine I get:

  AVERAGE       MAX       MIN
------------------------------
64.994300 65.748000 64.236000: ~/.config/nvim/init.lua
 8.342700  8.888000  7.954000: ~/.local/share/nvim/site/pack/paqs/start/onedark.nvim/colors/onedark.lua

and after:

  AVERAGE       MAX       MIN
------------------------------
60.578100 62.654000 59.836000: ~/.config/nvim/init.lua
 6.268300  6.810000  5.928000: loading packages
 3.631400  3.860000  3.472000: loading rtp plugins
 2.773000  2.908000  2.604000: reading ShaDa
 2.659900  2.859000  2.508000: ~/.local/share/nvim/site/pack/paqs/start/onedark.nvim/colors/onedark.lua

[cljoly]

The latest commit will need to be squashed if this PR is merged, but in the meantime, it makes testers live easier because they can just pull from the branch.

fmt was incorrectly handled previously, specifically the iteration was incorrect in Lua. The latest commit fixes that.

[cljoly]

After daily driving this change, I have not noticed any more discrepancies compared to master.

[navarasu]

This plugin initially started with nvim_set_hl. But we switched to vim highlight to solve some color highlight issue. Let me check that once

[cljoly]

Looking at the 0.7 changelog, it seems that nvim_set_hl has been significantly improved, so hopefully that’ll work now.

[navarasu]

Sure. Let me verify it this week and will merge if no issues

[cljoly]

Cool, let me know if you need help!

[cljoly]

Did you encounter any issues in the end?

[cljoly]

ping @navarasu , in case the notification just got lost.

[cljoly]

Rebased on the most recent master branch.

[carmen-gh]

let's get this merged...

[abhinavnatarajan]

Bump!