Fiber context leakage

[tsahara]

Creating many Fiber objects (then freeing them by GC) causes memory leakage:

% cat f.rb
100.times { |n|
  10000.times { |i|
    f = Fiber.new {}
    f.resume
  }
  puts n
}

% bin/mruby f.rb
0
1
2
3
4
5
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7
8
9
10
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23
trace:
        [4] f.rb:3:in Object.call
        [3] /home/tsahara/src/mruby/mrblib/numeric.rb:77:in Integral#times
        [2] f.rb:2:in Object.call
        [1] /home/tsahara/src/mruby/mrblib/numeric.rb:77:in Integral#times
        [0] f.rb:1
RuntimeError: Out of memory
zsh: segmentation fault (core dumped)  bin/mruby f.rb

I found that Fiber's contexts (f->cxt) were freed in obj_free before the commit 3531fe1 and the commit removes mrb_free_context() call (but I don't know how to free Fiber objects correctly...).