fix RubyFile#rename when the source is a broken symlink
[Truffle] Remove last instances of CoreLibrary.isTruthy.

* Introduce PredicateDispatchHeadNode which is simply
  BooleanCastNode(DispatchHeadNode)
[Truffle] Remove hard-coded defined? values.
[Truffle] Unify the core routine for match method and operator.
[Truffle] Tidy up where regexp match variables are defined.
[Truffle] Last of the Regexp defined? specs.
typed_enums
fix specs so they run
[Truffle] Let blocks know their declaring module and user that in super.
Merge pull request #2274 from tduehr/typed_enums

Typed enums
Merge pull request #2273 from lumeet/master

fix RubyFile#rename when the source is a broken symlink. Thanks @lumeet.  Great to have you on board.  I will evaluate whether this should be backported to 1.7 branch as well.
Speculatively untag from merge of #2273.  Fixes #2153.
[Truffle] Last of the defined? specs.
[Truffle] Last of the while specs.
[Truffle] Bunch of specs that already pass.