refactor(luasnip): minor code improvements and cleanup

[soifou]

- Cache module to avoid repeated require calls

• Use more efficient table insertion methods
• Remove dups in cached snippets introduced in commit #712af9f
• Improve code readability

[Saghen]

The require('luasnip') calls should be free after the first call: https://www.lua.org/pil/8.1.html. Do you have before and after numbers for the table insertion?

[soifou]

That description was "click-baity" I owe you that, I updated the message to something more in phase. I know the subsequent require calls take no costs, that was mostly for readability.

The removal of dups + table lookup when inserting items was the motivation for using a "perf" word but I was more concerned by the dups.

As for the numbers, this is mostly negligible, I did a quick n' cheap bench on my machine with 20 iterations on 2 runs, master vs this branch (added extra 10ms to get something readable).

local time_elapsed = os.clock()
-- source:get_completion()
vim.uv.sleep(10)                                              
time_elapsed = os.clock()-time_elapsed                        
vim.print(("Completed in %.5f seconds"):format(time_elapsed))

Results

# Master (run 1)
Completed in 0.00133 seconds
Completed in 0.00112 seconds
Completed in 0.00166 seconds
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Completed in 0.00055 seconds
# Run 2
Completed in 0.00102 seconds
Completed in 0.00055 seconds
Completed in 0.00133 seconds
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# This branch (Run 1)
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# Run 2
Completed in 0.00118 seconds
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[soifou]

Closing this PR since the duplicate items has been resolved in HEAD and the remaining changes debatable.