Mark and sweep gc implementation.

[Pavel-Durov]

Implemented mark-and-sweep GC as a non-moving, lazy, page-based collector.

On a 1MB heap it runs 1.03x – 1.51x the time of COPYING on the AWFY benchmarks.

The previous implementation (now preserved as DEBUG_MARK_SWEEP) was correct but slow - 1.36x–3.71x compared to COPYING on AWFY because it malloc/free every object, see #88.

AWFY benchmarks with default heap - 1MB

MARK_SWEEP runs 1.03x - 1.51x the time of COPYING.

confidence level: 99%

 Benchmark             COPYING (ms)  MARK_SWEEP (ms)  Ratio  Summary
 List/csom/1500         24823 ± 14    25467 ± 82   1.03  2.59% slower
 Richards/csom/100      70722 ±182    74650 ±214   1.06  5.55% slower
 Towers/csom/600        24359 ±121    27390 ±184   1.12  12.44% slower
 Queens/csom/1000       37391 ±147    42600 ±273   1.14  13.93% slower
 Bounce/csom/1500       30493 ± 87    35240 ±239   1.16  15.56% slower
 Permute/csom/1000      31329 ±471    36974 ±823   1.18  18.02% slower
 Sieve/csom/3000        52799 ±187    63303 ± 98   1.20  19.89% slower
 NBody/csom/250000      29889 ± 28    36533 ±148   1.22  22.23% slower
 Mandelbrot/csom/500    20259 ±438    25014 ±621   1.23  23.47% slower
 Json/csom/100          20732 ±276    26293 ±111   1.27  26.82% slower
 CD/csom/250            53464 ± 74    70570 ±425   1.32  31.99% slower
 Havlak/csom/1500       52799 ±666    70087 ± 79   1.33  32.74% slower
 Storage/csom/1000      27450 ±102    36770 ±158   1.34  33.95% slower
 DeltaBlue/csom/12000    8495 ± 16    12847 ± 64   1.51  51.22% slower

Comparing with a larger heap - 32MB

Here MARK_SWEEP runs 1.06x - 1.33x the time of COPYING.

confidence level: 99%

 Benchmark             COPYING (ms)  MARK_SWEEP (ms)  Ratio  Summary
 List/csom/1500          24445 ± 23    25921 ± 29   1.06  6.04% slower
 Richards/csom/100       64818 ± 72    75303 ±190   1.16  16.18% slower
 Towers/csom/600         23527 ± 53    28636 ±112   1.22  21.71% slower
 Havlak/csom/1500        52496 ±670    64347 ±270   1.23  22.57% slower
 CD/csom/250             43901 ± 45    53909 ±120   1.23  22.80% slower
 Json/csom/100           17387 ±113    21595 ± 47   1.24  24.20% slower
 Queens/csom/1000        35887 ±128    44660 ±306   1.24  24.44% slower
 Sieve/csom/3000         49827 ±114    62703 ± 43   1.26  25.84% slower
 DeltaBlue/csom/12000     8381 ± 39    10582 ± 57   1.26  26.27% slower
 Bounce/csom/1500        28609 ±214    36294 ± 93   1.27  26.86% slower
 NBody/csom/250000       28974 ±110    36908 ± 67   1.27  27.39% slower
 Mandelbrot/csom/500     19619 ±453    25076 ±691   1.28  27.81% slower
 Permute/csom/1000       29757 ±244    39437 ±561   1.33  32.53% slower
 Storage/csom/1000       22347 ±376    29783 ±185   1.33  33.27% slower

[Pavel-Durov]

CI failures are due to networking error:

...
failed: Temporary failure in name resolution.
wget: unable to resolve host address ‘apt.llvm.org’