Compatibility for Thread._locks

[thewilsonator]

See dlang/druntime#2519

[thewilsonator]

Hmm, this doesn't work: https://issues.dlang.org/show_bug.cgi?id=19761

[Shachar]

The whole point of using this method was to grant us access to private members which we need in order to switch fibers not from within the Thread druntime module.

The real solution to this is to create an advertised API in Thread for switching fibers, and have D's Fiber class use that instead of accessing private members of Thread (there is no reason for Fiber and Thread to share a module aside from this).

This would allow Mecca to use public APIs for switching fibers.