Enhancement: add trait getPointerBitmap to help precise scanning

This helps the precise GC (https://github.com/D-Programming-Language/druntime/pull/1022) to generate the required data without adding to the compile time.

I have copied that from another test, but I shouldn't have used traits_getUnittests.d for that :-)
The bad thing about main() and building without -unittest is that asserts are a noop for all the -release builds.