Define AST module under RubyVM [experimental] * ext/-test-/ast/ast.c: Rename to ast.c and define AST module under RubyVM. * common.mk: compile ast.c. * ext/-test-/ast/extconf.rb: Don't need this file anymore. * inits.c (rb_call_inits): Call Init_ast to setup AST module. * test/-ext-/ast/test_ast.rb: Follow up the namespace change.