Add explicitCast helper to std.conv

Makes possible to define more restricted cast that won't accidently
result in broken behavior during refactoring.

As discussed in http://forum.dlang.org/post/m7datc$2p31$1@digitalmars.com
Constraint will result in even worse error message (and there is not possibility for overloading). I can add static assert but it will say roughly the same.