More embarrassing microbenchmars for D's GC.

[Rounin]

I found this posting after investigating a huge slowdown in a D program I'd made
using gprof, which showed the culprit to be a function called
_D3gcx3Gcx4markMFPvPvZv, in which the program spent 87% of its time. That turned
out to be the garbage collector.

The program takes 1.74 seconds to run with two particular input files in Python.
The garbage collected D version takes 5.27 seconds.
The non-garbage collected version takes only 0.68 seconds.

Here's a more detailed report:

http://rounin.livejournal.com/21815.html

Obviously, the compiled program should run at least as fast as Python, so the
garbage collector could use some improvement.