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.

Fix more integer overflows.

Reported by https://hackerone.com/haquaman

This is to avoid segfault when WORD_BOXING is enabled

Reported by https://hackerone.com/brakhane

Reported by https://hackerone.com/haquaman

Testcase (couldn't get it to work as a test):

def nil.b
  b *nil
end
nil.b

Feature/multi unicode escape

Add test for recently fixed bugs