Define new at-generate_jll_init macro to replace hardcoded __init__

[giordano]

In case in the future we will have a different mechanism than __init__. This also makes #54 easier, the wrapper footer could be included directly into this macro.

[codecov]

Codecov Report

Merging #55 (173e727) into master (5b14792) will increase coverage by 0.81%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   54.05%   54.86%   +0.81%     
==========================================
  Files           7        7              
  Lines         222      226       +4     
==========================================
+ Hits          120      124       +4     
  Misses        102      102              

Impacted Files	Coverage Δ
src/wrapper_generators.jl	67.74% <100.00%> (+4.77%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[IanButterworth]

Seems like a good idea towards flexibility.

Beyond this, could a JLL be defined by a set of TOMLs of products etc. for each platform and the package just be something like

module libfoo_jll

@setup_jll

end

Where @setup_jll finds the right TOML for the platform and does what it needs to with the products?