remove redundant BlkAttr declarations

- use core.memory.GC.BlkAttr
- add private BlkAttr alias to replace declarations
- replace BlkAttr.ALL_BITS with ~BlkAttr.NONE
Merge pull request #634 from dawgfoto/blkAttr

remove redundant BlkAttr declarations
use int instead of bool for C function rt_unloadLibrary

- This should actually be ABI compatible but it's cleaner
  for documentation purpose.
Merge pull request #623 from dawgfoto/rt_loadLibrary

use int instead of bool for C function rt_unloadLibrary