for-in construct

[hishamhm]

Just pushed to the server some code I had lying around when I started playing with adding for-in. So far it has just the parser update and beginnings of type checking.

Evidently this needs to wait until we have other features in place for the complete functionality of for-in to work (first class functions being the most obvious one) unless we want to support a hacky version first with hardcoded support for the standard library iterators only (which I see as being special-cased in the code-generator anyway, so it's not like that would be throwaway code).

[mascarenhas]

Having the possibility for top-level functions being first-class functions, which is enough for Lua's stateless iterators, is not hard, as they are literally function pointers. But to really be convenient we also need to have parametric polymorphism, otherwise you need to have a different ipairs for each type of array element.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@d35ca06). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master      #42   +/-   ##
=========================================
  Coverage          ?   81.17%           
=========================================
  Files             ?       16           
  Lines             ?     2842           
  Branches          ?        0           
=========================================
  Hits              ?     2307           
  Misses            ?      535           
  Partials          ?        0

Impacted Files	Coverage Δ
titan-compiler/parser.lua	93.45% <ø> (ø)
titan-compiler/ast.lua	93.05% <100%> (ø)
spec/parser_spec.lua	100% <100%> (ø)
titan-compiler/checker.lua	76.5% <9.09%> (ø)

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