Use mrb_bool for the 'b' format specifier of mrb_get_args

Add Symbol#to_proc, Symbol.all_symbols

Minor cleanup

Cleanup around mrb_code, mrb_aspec.

Cleanup string.c 20130421

Implement respond_to_missing?

Refactor kernel.c and class.c

tiny fix on parser.y: change isxdigit to ISXDIGIT

Add mruby-string-ext methods

Make str_modify public

Add mruby-proc-ext

Minor format changes and a typo fix

Rollback stack before executing ensure clause.