Replace `//printf` with `debug(cast_) printf` in `rt.cast_`.
Refactor `rt.cast_`.
Fix `_d_interface_cast` and `_d_dynamic_cast` function signatures.

Both may return either Object or an interface.
remove unreferenced code
remove no longer called functions
Merge pull request #419 from WalterBright/fixtrace

remove unreferenced code
Merge pull request #420 from WalterBright/fixllmath

remove no longer called functions
Merge pull request #417 from denis-sh/rt.cast_-small-improvements

`rt.cast_`  small improvements