Implement a proper garbage collector

[pvginkel]

The current garbage collector implementation is far from ideal. The LuaJIT garbage collector looks like it would be relatively easy to implement. See https://github.com/LuaDist/luajit/blob/master/src/lj_gc.c for the code for this.

[pvginkel]

Also see http://jayconrod.com/posts/55/a-tour-of-v8-garbage-collection for how V8 implements the garbage collector.

[pvginkel]

A number of tests are currently suppressed either because they are too slow with the current garbage collector. After implementing a proper garbage collector, these tests must be revisited. See the test262-ignore.json for references to this issue for applicable tests.