chore(gen-*): Move templates into external files

[abhinav]

This moves the templates used by gen-atomicint and gen-atomicwrapper
into external files embedded into the corresponding gen binary with
go:embed.

[codecov]

Codecov Report

Merging #121 (38fd468) into master (9680036) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #121   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines          231       231           
=========================================
  Hits           231       231           

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[rabbbit]

Out of curiosity, I understand what we did, but why did we do this?

Are there any benefits outside of general code hygiene?

[abhinav]

Just hygiene. Easier to maintain the template if it's in an external file.
I'd already made the change when I was investigating a possible fix for #120 before we decided not to.
But since the cleanup was already done, I figured it was worth sending it out.