[xmlp] the recent garbage collector performance improvements

[Richard Webb]

Last night I tried loading a ~20 megabyte xml file using xmlp (via the
DocumentBuilder.LoadFile function) and a recent dmd build, and found that it
took ~48 seconds to complete, which is rather poor.
I tried running it through a profiler, and that said that almost all the
runtime was spent inside the garbage collector.

I then tried the same test using the latest Git versions of dmd/druntime (with
pull request 108 merged in), and that took less than 10 seconds. This is a
rather nice improvement, though still somewhat on the slow side.

Some profiler numbers, if anyone is interested:

Old version:
Gcxfullcollect: 31.14 seconds, 69.26% runtime.
Gcxmark: 4.84 seconds, 10.77% runtime.
Gcxfindpool: 2.10 seconds, 4.67% runtime.

New version:
Gcxmark: 11.67 seconds, 50.77% runtime.
Gcxfindpool: 3.58 seconds, 15.55% runtime.
Gcxfullcollect: 1.69 seconds, 7.37% runtime.

(Assuming that Sleepy is giving me accurate numbers. The new version is
definately faster though).