* Changed top-level exception class name from 'Exception' to 'Throwable'.

* Created a new class 'Exception' which derives from 'Throwable'.
* Created a new class 'Error' which derives from 'Throwable'.
* Moved core modules from the top level into a package named 'core'.
* Added onHiddenFuncError() routine to pass hidden function error handling from the runtime to core.exception.
* Renamed _d_getErrno to getErrno and moved it to src/common/stdc.  The idea is that druntime may eventually generate a lib for the stdc modules, and this definition is necessary.  The comparable function will have to be removed from Phobos as well.
* Moved the GC code into a package named 'gc', which is in accordance with the spec defined for druntime library integration (as yet unpublished).

git-svn-id: http://svn.dsource.org/projects/druntime/trunk@29 4a9d5153-6564-4b3f-b5e1-7e8e9dac548f