DIP53 and DIP49 (ver2) - New definitions for qualified constructors and postblits

[Kenji Hara]

http://wiki.dlang.org/DIP53
Redesign currently implemented qualified constructor concept.

http://wiki.dlang.org/DIP49
Improved points from version 1:
- Swap meanings of 'this(this) inout' and 'this(this) const'
- Inout postblit now covers all cheap (== not rebind 
indirections) copies between same qualifiers

Kenji Hara