Refactor ast compilation

[arnodel]

ast -> ir compilation is refactored into the astcomp package, to remove the dependency ast -> ir. Now we have:

ast <- astcomp -> ir

[codecov]

Codecov Report

Merging #34 (6f2d298) into master (2a16464) will decrease coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   89.59%   89.00%   -0.60%     
==========================================
  Files          70       70              
  Lines        5893     5303     -590     
==========================================
- Hits         5280     4720     -560     
+ Misses        419      390      -29     
+ Partials      194      193       -1     

Impacted Files	Coverage Δ
runtime/lib.go	75.42% <100.00%> (-2.97%)	⬇️
runtime/cont.go	16.66% <0.00%> (-13.34%)	⬇️
runtime/error.go	44.44% <0.00%> (-9.13%)	⬇️
lib/base/dofile.go	42.85% <0.00%> (-7.15%)	⬇️
lib/golib/golib.go	51.51% <0.00%> (-5.82%)	⬇️
luatesting/linechecker.go	72.41% <0.00%> (-4.73%)	⬇️
runtime/numconv.go	61.29% <0.00%> (-3.42%)	⬇️
lib/iolib/file.go	74.28% <0.00%> (-3.37%)	⬇️
lib/base/ipairs.go	63.63% <0.00%> (-3.04%)	⬇️
... and 61 more

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