Some more kwargs error work++
post_reqd_arg already knows about kwargs
Refactor optarg logic (no fixes)
Another round of kwargs refactoring
Receive arguments know enough to extra proper kwargs value itself (fixes issue that interpreter did not know 100% if last hash args was a kwarg or not
fix tyop
remove dead code
[Truffle] - Using putIfAbsent to add a new RubySymbol in the SymbolsTable.

Thanks @thedarkone to help us with that. Without putIfAbsent multiple
threads might concurrently call createSymbol with the same String name,
create a bunch of RubySymbols, store them in symbolsTable (overwriting
each other) and then return them to rubyland. But using putIfAbsent we
avoid this behaviour.