Revert "[Truffle] Top-level methods should be public by default."

* This reverts commit 6be4fe268863bfdcbcc0e7a728f49223b4c4d1ba.
* They should be private: $ ruby -e 'def m; end; p self.private_methods(false)'
Geez. Make a more realistic error message
Remove newline unwrapping because I cannot get any tests to fail with it gone
[Truffle] Set top-level visibility only in RubyContext.load and TruffleBridgeImpl.execute.

* Since the meaning of ParserContext.TOP_LEVEL is unclear.
* De-duplicate one call to RubyContext.execute.
enumerator, rational, and complex are considered loaded at boot.
Next parser generation will pick up these comments but they are only for my notes
All ArrayNodes for args list are lightweight.  Simplify buildCallArgs a little more
Remove some arity-split AST nodes which we no longer use
Remove lightweight flag from nodes since we no longer use that information
Merge branch 'master' into truffle-head