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

Copy default_proc by Hash#dup.

Removed the errno declaration from string.c

set `MRB_64BIT` if the sizeof(size_t) is 8

The function used to return NULL on error, but not checked in the
caller site.

Fix stack move segfaulting in OP_ARYCAT

The following codes used to be SyntaxError:

(1)
  a = <<-EOD;
  hello
  EOD

(2)
  <<-EOD.bla begin
  k
  EOD
  end

The function was renamed to `mrb_float_read(const char*, char**)`.

Merge branch 'method-missing-segfault' of https://github.com/bouk/mruby into bouk-method-missing-segfault

Fix segfault when using result of rest assignment

range->edges may be NULL for example when #initialize_copy removed.

Use mrb_ptr instead of mrb_cptr in Kernel#to_s

Fix some incompatibility for Enumerator#with_index

Fix segfault in mrb_proc_copy.