refactored error message apis
adding @ZenMethod
adding @ZenIgnored
updated sample/libzen.zen
update JSSourceGen
add VisitLetNode to JSsrcGen
fix extends in JSsrcGen
enabling prototype
bugs fixed at NullPointerException is found
Merge branch 'master' of github.com:konoha-project/libzen
bugs fixed at AssertNode
bugs fixed at passing parser
bugs fixed at JavaScriptSourceGenerator
trying self-hosting