Fix crash when exponent is -2147483648

Check type before calling mrb_range_ptr.

Reported from @clayton-shopify.

Also msym size changed to 512 from 256.

Support pkg-config in mrbgem.rake

Implement Object#freeze

Example usage:

    MRuby::Gem::Specification.new('mruby-onig-regexp') do |spec|
      # ...
      if spec.search_package('onigmo')
        # Use onigmo.h when onigmo.pc exist.
        spec.cc.defines += ["HAVE_ONIGMO_H"]
      elsif spec.search_package('oniguruma')
        # Use oniguruma.h when oniguruma.pc exist.
        spec.cc.defines += ["HAVE_ONIGURUMA_H"]
      else
        # Use bundled Onigmo otherwise.
        # ...
      end
    end

Mark all the built-in classes during GC sweep

Don't generate code for NODE_NEGATE if the result isn't used
Reported by https://hackerone.com/haquaman

do not destroy a page with an active TT_ENV

Fix segfault in gen_values with NOVAL and more than 127 args

Fix segfault when undef is called with more than 126 arguments

The issue is that when there are more than 126 arguments an array needs
to be created to pass the arguments on with.

Reported by https://hackerone.com/revskills

Removed unnecessary const macro - const keyword is already a dependency

fix build on vs2013-vs2015

Promote load_exec to mruby API as mrb_load_exec (fixes #3248)

fix issues of mrb_gc_unregister introduced in 09b11850

* fixes partial copy of objects in GC root array (due to missing `* sizeof(mrb_value)`)
* restores the behavior that permitted an unregistered object to be used as an argument