Remove the mutex protecting tick_event_queue, As 3.0 have no multi-threading planned.
Merge event.cc into backend.cc

Move some VM related stuff into vm package.
Fix crasher in overriding undefined function by inherit.

Fix #221.
Reformat bison grammar file.

Also let bison auto generate grammar report, and enabled bison warning.
Merge pull request #222 from sunyc/master

Continuing code cleanup