Implement `eval("...")` in the StdLib

[Jason Steving]

I'll be honest....I think that this will be where Claro jumps the shark and I'm pretty likely to consider remove its support before ever actually trying to release Claro as something that I think real people should be using... but in the meantime it'll be seriously dope to see this in action dog-fooding the interpreted compiler backend within the StdLib of the JAVA_SOURCE backend, and even somewhat recursively through the REPL backend!

It'd be cool to be able to do something like:

eval("
var s = input(\"hey gimme some input\");
print(s);
");