Provide simple runtime-enabled D version
I went through translating the Java version to simple, straightforward D with runtime and GC. Compiled with DMD on my 2013 Macbook Pro with a dual-core i5, I get about 0.61s - 0.65s real runtimes.

This one is simple enough to be used with `rdmd`:

`rdmd treap_gc.d`
`time rdmd treap_gc.d`

Remember that the first `rdmd` run compiles, so it will take longer.

With -O:

`rdmd -O treap_gc.d`
`time -O rdmd treap_gc.d`

times reduce to about 0.46s - 0.48s.

[treap_gc_d.tar.gz](https://github.com/frol/completely-unscientific-benchmarks/files/2013676/treap_gc_d.tar.gz)

Feel free to compile with LDC and all sorts of options when testing.

Edit: Apologies, I see there's already a runtime-enabled version, but I don't see the code that was used. I'll leave this issue up in case it proves useful.