Implement `LVAR` section remover in 'mruby-strip'.

* Added API `mrb_irep_remove_lv()` to remove all local variables information from irep tree.
* Checks irep tree's local variables information before dumping `LVAR` section.
* `LVAR` section remover could be used by passing `-l` or `--lvar` option to **mruby-strip**.

**mruby-strip** has new dependency **mruby-proc-ext** since it use `Kernel.local_variables` in test.
(`Kernel.local_variables` should be moved to core in future since it's defined in ISO.)

The same feature should be implemented to **mrbc** too.