[Truffle] Reuse SingletonClassNode in Kernel#singleton_class.
[Truffle] The path in $" should only be expanded, not canonicalized.

* See https://jira.codehaus.org/browse/JRUBY-6805.
Remove no longer used constructor
Fail fast if we see anything other than the four expected node types in call args
[Truffle] Top-level methods should be public by default.
[Truffle] Added String#concat.
[Truffle] Untagged some passing String#concat specs.
Merge branch 'master' of github.com:jruby/jruby