num: implement CheckedDiv
Ported check_const from oldvisit to <V:Visitor> trait API.
Ported check_loop from oldvisit to <V:Visit> trait API.
Port check_match from oldvisit to <V:Visitor> trait API.
Port const_eval from oldvisit to <V:Visitor> trait API.
Ported effect checker from oldvisit to <V:Visitor> trait API.
Port privacy.rs from oldvisit to <V:Visitor> trait.
port reachable.rs from oldvisit to <V:Visitor> trait.
port region.rs from oldvisit to <V:Visitor> trait API.
port resolve.rs from oldvisit to <V:Visitor> trait.
Ported trans/base.rs from oldvisit to <V:Visitor> trait API.
Ported liveness.rs from oldvisit to <V:Visitor> trait API.
Ported typeck::collect from oldvisit to <V:Visitor> trait API.
port callee.rs from oldvisit to <V:Visitor> trait API.
port type_use.rs from oldvisit to <V:Visitor> trait.
Ported typeck/check/mod.rs from oldvisit to <V:Visitor> trait API.
Port regionck from oldvisit to <V:Visitor> trait API.
Fix select deschedule environment race for real this time, in light of task killing.
Cleanup: remove useless parameter
Test for issue #6338.