Reduce compilation time for phobos unittests by about 25%

Wondering why building the phobos unittests take so long, I broke into dmd during compilation and noticed that it was optimizing huge functions. These are created by nested loops over TypeTuples expanding to enormous LOCs.
This PR makes most nested foreach loops over TypeTuples into calls to lambdas to avoid these huge functions. For me, it reduced compile+execution of the unittests from 3:30min to 2:40min.