Added a modified version of core.demangle to get proper type demangling.

The solution is temporary until
https://github.com/D-Programming-Language/druntime/pull/434
is merged, or another replacement is available.

This fixes the issue mentioned at
https://github.com/D-Programming-Language/d-programming-language.org/pull/267#issuecomment-14538594
Demangled string types are replaced by their standard aliases now.