The following error occurs when using `rake -m` on AppVeyor:

  ```
  fatal error C1041: cannot open program database 'C:\projects\mruby\vc140.pdb'; if multiple CL.EXE write to the same .PDB file, please use /FS
  ```

Therefore, the issue is solved by not creating the PDB file. It is expected
to be somewhat faster by not generating debugging information (I don't think
debugging information is necessary for normal CI).

This reverts commit ac68c908.
`rake -m` does not work for VisualC.

Adding warnings for mrb_load functions leaking RProc objects

Unify `eql?` receiver in `Hash` according to Ruby

### Example

  ```ruby
  # example.rb
  class A
    def eql?(o)
      p self.class
      super
    end
    def hash
      1
    end
  end

  class B < A; end

  h = {A.new => 1}
  h[B.new]
  ```

#### Before this patch:

  ```console
  $ bin/mruby example.rb
  A
  ```

#### After this patch (same as Ruby)

  ```console
  $ bin/mruby example.rb
  B
  ```

Instead we added `%define parse.error verbose`.

The newer `clang` warns implicit float conversions.

Get an irep symbol if it's `OP_GETSV` or `OP_SETSV`

Update `doc/opcode.md` [ci skip]

The difference of `include/mruby/ops.h` is applied.

- OP_NOP - update semantics
- OP_GETSV - update semantics
- OP_SETSV - update semantics
- OP_GETUPVAR - update prefix
- OP_SETUPVAR - update prefix
- OP_JMPIF - update operands and semantics
- OP_JMPNOT - update operands and semantics
- OP_JMPNIL - add entry
- OP_ONERR - update semantics
- OP_POPERR - update prefix
- OP_EPOP - update prefix
- OP_SENDB - update semantics
- OP_ADD - update prefix and operands
- OP_ADDI - update operands and semantics
- OP_SUB - update prefix and operands
- OP_SUBI - update semantics
- OP_MUL - update prefix and operands
- OP_DIV - update prefix and operands
- OP_EQ - update prefix and operands
- OP_LT - update prefix and operands
- OP_LE - update prefix and operands
- OP_GT - update prefix and operands
- OP_GE - update prefix and operands
- OP_ARYDUP - add entry
- OP_INTERN - add entry
- OP_HASHCAT - add entry
- OP_ERR - update semantics

Remove byteorder constants; ref 87576b81

Fix boundary check for `OP_LOADI16`; ref fa8668c7

The `FLAG_BYTEORDER_NATIVE` and `FLAG_BYTEORDER_NONATIVE` are no longer
needed.

It was making a negative integer if the highest-order bit of a 16-bit
integer was 1.

no patched:

```ruby
p 0x7fff  # => 32767
p 0x8000  # => -32768
p 0xffff  # => -1
p 0x10000 # => 65536
```

Fix some `Hash` methods are inconsistent with `values`

Inconsistent when hash has duplicate key.

### Example

  ```ruby
  # example.rb
  keys = (1..3).map{[_1]}
  h = keys.to_h{[_1, _1[0]]}
  keys[0][0] = 2
  p h.values
  p h.each_value.to_a
  p h
  ```

#### Before this patch:

  ```console
  $ bin/mruby example.rb
  [1, 2, 3]
  [1, 1, 3]
  {[2]=>1, [2]=>1, [3]=>3}
  ```

#### After this patch (same as Ruby)

  ```console
  $ bin/mruby example.rb
  [1, 2, 3]
  [1, 2, 3]
  {[2]=>1, [2]=>2, [3]=>3}
  ```

`cmpnum` function may return `nil` on error.

Since `mruby 2.0`, compiled bytecode no longer depends on the
endian of the machine.

You don't have to specify endian since `mruby 2.0`.

- Add `OP_LOADI16`
- Update keyword argument related ops (e.g. OP_KEY_P, etc.)
- Fix some wrong descriptions

Which loads 16bit integer to the register. The instruction number should
be reorder on massive instruction refactoring. The instruction is added
for `mruby/c` which had performance issue with `OP_EXT`. With this
instruction, `mruby/c` VM can just raise errors on `OP_EXT` extension
instructions.

`mrb_run` requires to push callinfo stack before calling, which is very
hard from outside of `vm.c`. So there should be virtually no correct
usage of the function, hence the cause of #4488. We removed it.

You can use `mrb_top_run(mrb, proc, self, 0)` instead of
`mrb_run(mrb, proc self)`.

Enable MRB_METHOD_T_STRUCT by default on 32bit GUN/Linux

Because we can't use the highest 2 bits of function pointers.

Avoid changing directory in `mruby-io` test

Refine `.travis.yml`

* Disable automatic update and clean up on `brew install` (install time
  160 sec -> 5 sec).
* Avoid using deprecated keys.